A question about returning a recent upgrade/reserving?

[Patient]

Hi. I realize there have been a lot of similar questions to mine, but I hope someone here can help me out since I'm fairly new with dealing with Apple and the iPhone in general. I upgraded to a 3GS on Saturday so that I could be grandfathered in for the unlimited data plan. The AT&T store I purchased it from told me I have 30 days to return it and get my original upgrade status back. I plan on returning it for an iPhone 4 since several AT&T employees have told me that I could keep the unlimited data plan if I did this. Does anyone know if this is true? Also, would it be possible for me to still reserve a phone on the 15th even though I don't have upgrade eligibility and will be returning my phone for it? If not I suppose I'll have to show up pretty early. I have to be able to get the iPhone 4 within 30 days from Saturday and am worried about stock issues. Thanks for any advice!