The problem that I'm having with this news is that the audience today is a lot more advanced than they were 40 years ago. On the one hand, this means that people will generally understand that these are TV shows and they'll roll with some punches (Superman appearing on TV and in films, played by two different actors, in stories that are completely unrelated to each other). On the other hand, it means that you really can't pull off a recasting without it being a brutal and jarring transition.
Casting Erica Durance on Supergirl at all is a nice way to show respect to what came before, but it will also have people thinking about Smallville and her Lois Lane nonstop. It would be hard for her to disappear into any role on that show, but it could probably happen over time. In theory. Personally, I think she's still a little too close to the Superman world right now, but that's just me.
Now, using Erica Durance for such a major recast is like a four car pileup of distractions that can't possibly help sell the story. It's like the producers created a diagram of just how rough they could make the transition for this character, and made sure to hit each point.
The thing is, they don't need Alura or Astra. I know that it was probably their ideal situation, but it couldn't work out, and no matter what they were planning, they could have gone another way. Focus on Kara's father this year. Or bring in Lara instead of Astra. Or have Clark convey the story. Or... whatever. This isn't a train that had already left the station and couldn't be rerouted. It was a train that they knew was heading for incomplete tracks, but they decided to go with it anyway. And this is a symptom of a much larger problem with the series, which is why I can't watch it anymore. They don't care about building a world or telling a story. They're not taking any of it seriously. They're delivering some half-assed garbage week after week, which has more in common with a bad Saturday morning superhero series from the late 80's than it does with a primetime superhero series today.
They have access to great, iconic characters that most writers would kill to work with, and they don't even give a damn. It's irritating.
But I'm already done with the show. There's an alternate version in my head, which I hear is going to cross over with The Flash for a musical. It's going to be epic... but still in my head.