A rule of thumb:
Changes to existing traits, triggers, localisation and the creation of new triggers are save game compatible. I can personally confirm all the changes I've made in this thread are save-game compatible.
The creation and deletion of traits will break save games. I don't think changes to scripts actually take effect until a new game is started so they won't break your save game but they also won't do anything.
As for complexity: it's a great deal simpler than it looks: this is mostly for the modders' benefit (which now includes me I suppose haha). I could've simplified it (and indeed I will get around to creating something like a guide when I've implemented these changes in the mod) but I didn't really have time. What's important is the principles I lay out with respect to balance, historicity and interesting gameplay. All my changes respect that, and arguably the old system was more complex (by virtue of being poorly designed)