I last heard that Wallis Day was filming a movie in Tunisia and hasn't been anywhere near Vancouver in months. Narratively -- I think it makes sense to let Javicia truly be the lead of BATWOMAN for Season 3.
But looking back now -- I think that the writers probably could have transitioned from Rub Rose to Wallis Day in a Season 2 premiere.
They kept using Ruby Rose's photograph throughout Season 2. Since they had the rights to her image even if they didn't have Rose herself, they did have a way of transitioning from Rose to Wallis Day, but it would have been *extremely* difficult.
They could have rewritten the original Season 1 finale into a Season 2 premiere.
2.1- "The Death of Kate Kane"
Opening scene: 'Bruce Wayne'/Hush gets the Kryptonite bullet for Alice. He doesn't encounter Kate because Kate hasn't taken off the Batsuit or gone to Wayne Tower in weeks: she is relentlessly hunting the Arkham escapees.
When we see Batwoman, she's in full costume, masked, played by Kelli Victoria Scarangello (Ruby Rose's stunt double).

Kate doesn't speak without using the Batwoman voice changer and she refuses to take off the mask: she has buried herself in the Batwoman persona due to her grief over her father's betrayal of Batwoman.
When Jacob Kane calls Kate, she picks up but won't speak. Jacob says he knows she's angry at his public decision to hunt Batwoman down but says he's doing his job. Batwoman doesn't answer.
Batwoman is tracking down another Arkham escapee, but when the Crow agents try to arrest her, the escapee gets away. Batwoman is furious . Luke, Mary and Julia urge her to come back to the batcave, take off the costume and address the situation with her father, reveal her identity -- and Batwoman refuses, growling in her vocoder-deepened voice that the problem isn't Kate Kane and Jacob Kane. The problem is the Crows and the Crows are going down tonight.
At this point, Alice delivers the Kryptonite bullet to Jacob in her personal revolver of choice, remarking that Batwoman is their shared enemy.
Batwoman sets a trap for all the Arkham escapees, luring them with false messages saying that there is a stockpile of weapons in an old warehouse that they could use to rule the city. She leaks the messages to the Crows, lets the Crows and the escapees fight it out and batlines Jacob out of the fight.
She snarls at Jacob: who the hell does he think he is to come after her? She notes that below them, the Crows are losing against the escapees. Jacob spits that Batwoman lured his men into a trap; she's responsible for every Crow who dies below. We see Sophie in mortal peril below.
Batwoman triggers a flood of thickening foam that encases and immobilizes everyone on the ground floor, Crow and criminal alike.
She tells Jacob that the Crows are useless, the police are inadequate, Batman is gone: there's only Batwoman and the incompetent Jacob Kane. Jacob strikes Batwoman. Batwoman tries to defend herself but pulls her punches.
Jacob remarks that she seems to be a little soft today and Batwoman breaks his nose. She recoils, horrified and what she's done to her father. She tries to batline away, Jacob tackles her and her grappling hook punctures a gas line. She throws Jacob off, Jacob draws his standard sidearm, Batwoman throws off his aim and the shot triggers an gas fire.
Jacob descends to ground level to try to dig his people out of the hardened foam before the fire hits them. Batwoman sets off another chemical charge to desolidify the foam so that everyone can escape. But Jacob doesn't leave, pursuing Batwoman deeper into the burning building. They fight in the flames, Batwoman beats him down. From the floor, Jacob pulls out the revolver with the Kryptonite bullet and shoots to kill. Batwoman collapses. Jacob peels off her mask.
CUT TO: A shot of Ruby Rose unmasked, lying on the ground of the warehouse. This is unused footage from some previous episode I don't remember when Kate was unmasked and unconscious and in the suit. But surely it exists.
Jacob is horrified. Backs away several steps in shock and denial. It can't be Kate. It has to be a trick. "It can't be -- " he says. But then we stay on Jacob's face and hear Kate say, "Dad," in a reused audio clip from... again, I don't remember. But surely it exists.
Jacob turns to the warehouse exit, runs away from Kate and screaming that he needs help. Moving farther away from her, hoping his cries will be heard. Then he turns back towards Kate, starts to close the distance from them as she bleeds out. Jacob babbles that he's sorry --
But then a portion of the ceiling collapses between him and Kate. Jacob rushes forward anyway, but then arms grab him behind -- it's Sophie. He tries to fight her off but she drags him out of the burning warehouse and Kate just before the building is consumed in flame with Kate Kane inside.
2.2 - "Burial"
Kate's body is missing, never recovered from the wreckage. The city erupts into chaos without Batwoman and Jacob is forced to join the bat-team of Luke, Mary, Julia and Parker to contain the situation. They gather at Kate's funeral and Jacob begs God to forgive him.
However, at the end, we find out that a False Face Society member was trapped in the warehouse too; he dug Kate out; the Kryptonite bullet was partially deflected by Kate's lead-lined wrist-gauntlet, going through her body and missing any vital organs. She survived and was brought to Black Mask and is unrecongizably scarred and now played by Wallis Day.
2.3 - "Batwoman Reborn"
The city is attacked and Batwoman reappears to save the day -- but it's a Batwoman with a damaged costume and barely any of the red wig. Batwoman returns to the Batcave, surprising Julia, Mary and Luke. Julia says the real Batwomanis dead and fights this supposed-impostor, but recognizes that this Batwoman has the same fighting style as Kate Kane.
She unmasks and it's Wallis Day, whom the characters recognize as Kate. She looks a little different and all the tattoos are gone. All her skin has regrown new and shifted a little. They question her and she has all of Kate's memories up to the fire. Kate says she doesn't remember how she was healed from her injuries or why her face isn't quite the same.
We later learn that Black Mask healed her body and has programmed a sleeper personality into Kate's mind to be triggered at the right point in his plan for revenge against Batwoman and Gotham City.
2.4 - "Unworthy"
'Bruce Wayne' returns and accuses Kate of being an impostor with poor plastic surgery trying to steal the Wayne fortune. He has her ousted from Wayne Enterprises. He then tells Kate he knows she's the real Kate, but that she was a failure as a superhero in every way and that she is unworthy to be Batwoman.
He shuts her out of the cave; he files for damages against Kate for mishandling his company and triggers a clause in her contract which will drive her to bankruptcy in having to repay all of Wayne Enterprise's costs. Kate is shattered. All she has left is her damaged, burnt, ragged Batwoman suit.
2.5 - "Deficient"
A destitute and homeless Kate tries to keep fighting crime on a budget of $15 a week, wearing an increasingly worn Batwoman costume without any of her gadgets. Due to her injuries and missing memories, Kate is unable to summon her former drive for combat or aptitude for danger. She realizes she no longer has the skill to be Batwoman. Kate is nearly killed and then Jacob Kane saves her.
2.6 - "Prodigal"
Kate confronts her father, blaming him for destroying Batwoman. She has lost everything: she has even lost her face and her voice. Jacob says that he was wrong to hunt down Batwoman. Jacob says that he hated Batwoman because he didn't have the full story; he does now and he's proud of his daughter. Any father would be. Anyone would be proud for Kate to carry their mantle and name.
Kate regroups and realizes that the Wayne who disowned her and threw her out cannot be the real Bruce Wayne.
2.7 - "Unmasked"
Kate realizes that Hush is impersonating Wayne as part of a twisted mindgame from Alice. She defeats him and reclaims the right to wear the suit (and for Wallis Day to wear it after Ruby Rose). For the first time, Wallis Day suits up in full as Batwoman.
This way, we don't just slot Wallis into Ruby Rose's costume and pretend there's no difference. Instead, the Wallis-version of Kate has to go through six episodes of hell before she gets to wear it.
Episodes 8 - 16 have Kate continuing to crime crime and investigate her mysterious recovery. At times, she is haunted by images of a duplicate Kate (with her new face) mocking her, telling her she's not good enough. She also reveals to the public that due to a motorcycle accident, she's had to have reconstructive surgery and was misreported as deceased.
We resume the stories that we would have with the Rose-version of Kate. And in episode 17, Black Mask triggers the sleeper programming and 'replaces' Kate with the Cersei Sionis personality, the mocking shadow in her psyche who now becomes the dominant personality.
And in episode 18, Mary and Luke find a way to restore the full Kate personality and we go from there into Season 3.
**
That said, I really like Season 2 of BATWOMAN as it aired and Ryan Wilder is wonderful. And I don't find fault with the show for refusing to recast Ruby Rose in this manner and only being able to ease into it later and ultimately deciding to have Ryan be the star going forward.
It was an impossible situation and they handled it really well. The fact that they didn't handle it as I would have is not a valid criticism.