I was looking on club gp and found a ssite for aftermarket autmatic shifters. Well i hate my stock shifter and i was looking to get this.
http://www.mach3accessories.com/shift_knobs/sk_14.html

I was wondering what would be involved in replacing this and also how hard it would be.

Just do your research first! I've seen most Bonneville owners try these aftermarket shifter ***** WITHOUT ANY SUCCESS! You see, our shaft on the trans is thinner in diameter!

Not sure about the diameter and all that sutff, but I know replacing the handle is a breeze. Next time your in your car, look on the FRONT of your current handle, just underneath the trigger (towards the radio). You'll see a little slot, which has a clip in it that slides in and locks the handle on. Take a small flat-head screwdriver, and pry out that clip (dont loose it, its funky lookin and would be hard to replace), but after you pull that out, your handle will pull right up and off. Very simple, and our transmission shafts that stick up are actually slotted, so when yo go to put it on, it only slides down on 1 way.

Hope this helps, if not, talk to me in the chatroom later and I'll help ya out with some pics

You can make anything fit in any car, its all about fabrication, just because someone doesnt make it doesnt mean you cant make it fit. Get creative and try doing it yourself. Dont say never until you have proven through trial and error that it cannot be done.