Help using gophone on an iphone

[savage388]

Currently I've got a crappy go phone and an older iPod touch. After finding nice deals on iPhone 3GS's and iPhone 4's I want to combine them and upgrade but everywhere I've checked had a different thing to do and none gave a clear cut answer on how to get my sim switched and if data will work. Can anyone explain the steps to do it legitimately and will AT&T give me a hassle if I go up there? I've looked and none of this is illegal or in any sort of grey area, AT&T just seems to when people save money.