Sometimes, I realize I need to cut some characters or even combine characters or just change things. This results in scenes where I take out one character and use a different character instead. I did that once without needing to change the dialogue at all–and I realized I had a big problem. Dialogue should be distinctive to each character. Ideally, the reader should be able to tell who’s speaking without needing dialogue tags/attributions (but you should still include them). If you can substitute a different character without changing dialogue, there’s a problem.
Interchanging characters into the same scene can actually be a good way to get to know your characters. How would each character respond in this situation? How would they interact? I’ve been having fun putting the characters I’m keeping into the scenes formerly populated by the characters I’m cutting. It changes the dynamic. I like seeing how they interact.