I just went from stock Sprint to Mighty ROM the other night no problems. Easy enough to do. Follow this post for unlocking the phone:
http://forum.ppcgeeks.com/showthread.php?t=93252
Then follow the instructions in Mighty's first post. It's pretty simple though. After phone is unlocked, I backup my info, connect phone to PC and WMDC/Activsync, run the exe for the ROM, wait a few minutes and done other than restoring my contacts and programs, etc.