I just made the switch myself.
http://www.flyertalk.com/forum/Forum20/HTML/002057.html

I'm plat on AA and CO comped me gold in a matter of hours. CO's upgrade program far exceeds AA's. I just got an email confirming my upgrade for a flight tomorrow. I didn't have to ask for it and it didn't cost me any miles or coupons, they just did it. Enjoy!