What Changes Take Place When Installing A Custom Launcher eg Nova To A Rom?
If you install a alternate launcher, the first thing you'll see is a difference in how the home screen, widgets, (maybe lockscreen if it has support for that), and the app drawer appears and works.

AFAIK that's the only changes you'll see from changing the launcher regardless of what ROM you use.

But one thing's for sure - depending on which ROM you get, the experience will change very little or a lot.