problem with window jams

well i baught a house a few months ago, it needs a little work, its my first home so i am learning some of this as i go.

Well i have tore out plaster from the LR walls and ceiling, i did this because the plaster was in poor shape and there is not any insulation in the walls.
Well before the plaster was even tore out i noticed that the window jam ( the box that the window sits in ) was sticking out waaayy to far, now that i am ready for some drywall i need to trim this back so that it meets the drywall, i want it to stick out only 1/2" so it will be flush with the drywall, is there any way to cut the jam back with the windows in place?

it is thicking out about 1 1/2" right now.

thanks for the help.