Window Trim
 

Should the drywall be removed on the inside of the window to install jamb extensions? or can I go over the drywall?

Generally speaking drywall removal is preferred and will yield a better result, but there are some circumstances where it can be left in and look okay. Post up a pic if you would like some more specific feedback. :)

most windows aren't wide enough for the jambs to sit over the

drywall. you get a much better job taking the drywall and corner bead

off first esp if the corner bead isn't straight like a lot of them.

it also give you a chance to see if they insulated or foamed around the windows.