I have searched the forums but have not found a definite answer to this. If I make the change in Setup -> Domains to make newdomain.commy default, will that impact any of my currently configured Active Users? Or will their apps and accounts continue to function as expected? I believe I can do this with 0 effects, but I want to validate it, please.
You are correct, this will have no effect on your existing users. Any new users you setup as cloud only users will have this domain as their default, but you won't see any changes to existing accounts so you are good to go with this.