I changed out the whole doors on my 94 wrecked wagon, partly because the doors were damaged in the accident, and partly because i wanted the 95 doors from my parts car. If you're doors are not the same color, you are going to have to cut in the back side before you install. Also is alot easier if you have the front fenders off. I would say if the doors are not the same color, it would be easier to just change out the guts. Don't believe your harness is going to just plug in under the dash either. I went from manual seats to power, had a harness, but the plug under the dash is different, i just wired in my power wire to make it work, also changed from manual mirrors to power.
http://www.impalassforum.com/vBulletin/showthread.php?t=237761