If you change your common app, it will change for those you’ve not yet applied to.
If you have a significant change, then yes, inform the schools.
If it’s insignificant, like you put your religion but didn’t want to, I wouldn’t bother.
If you want to change your ED to EA or RD, then yes, find out how to do so. Of course, that might eliminate you altogether (or might not) - so be cognizant of that.
Good luck.