Do I have to restart?

Hi guys, I have a short question:

I just installed several mods to my BG2:EE (i.e. SCS, aTweks) - how do I know for what changes I have to start a new game for and which changes also affect existing savegames?
As I see this most changes also affect existing games but I read that for some changes to work I have to start a new game. How do I know which these are? Is there a rule of thumb?

Some changes affect the lategame and I don't want to notice that I should have restarted when I'm already in the underdark.