So, just before the finale...

[FlashStep]

I saved as Majima right after it asks me if I want to switch to him from Kiryu, which I did, and now I can't switch back. I was hoping to farm cabaret club/real estate for both chars to upgrade their movesets before the final fights. is this not possible?

[Duckilous]

It should be possible to switch back to Kiryu as long as you haven't sent him away with Nishiki.
I can't exactly remember but there should be an option to switch characters when you're at the point where you've been asked if you want to finish the finale now.