Recasting is a touchy subject in the Doctor Who fandom and that's a complex subject. Its very emotional and not entirely logical but I understand it. I can argue all day that there's no rational difference between Jemma Powell taking on the role of Barbara Wright that Jacqueline Hill originated and (to take a topical example) Kenneth Branagh being Poirot in the new Murder on the Orient Express. Branagh is Poirot now and that doesn't insult David Suchet's or Peter Ustinov's work in the role. Different actors play the same role all the time, its how acting works.
And yet I see the argument. Its purely emotional and there's nothing wrong with that. Jacqueline Hill is the Barbara Wright, Lis Sladen is Sarah Jane Smith. These were definitive performances that mean something to the fandom.
Personally I don't mind recasts but they will always be second best. I hate that I feel that because I feel like I'm insulting the new actors. Take Tim Treloar, for example, who has played the Third Doctor for Big Finish for a couple of years now. He puts in a fine performance and I include the fact that he explicitly plays a version of the Third Doctor and not a Jon Pertwee impression. From what little I've seen of David Bradley (in The Doctor Falls and clips of An Adventure in Time and Space) he seems to take a similar approach to the role (even when explicitly playing Wiliam Hartnell in the documentary).
I won't lie: I love what I've seen of Bradley and I think Big Finish are going to do well with this project but I say that with one caveat...
I think it would be better in the long run if they treated The First Doctor Adventures as effectively a reboot of Season One. This is an entire TARDIS team played by new actors, with a new Master as well, and I'd rather see the roles pushed in new directions for new actors than slavishly trying to imitate the past.
This is Big Finish so it could go either way but for the moment I think I'll tend towards the positive.