Very same thing happened to me last Monday. I spoke with an employee AND the manager and they both told me the same stupid thing.
To simply to everyone, YMMV on this. I've read plenty users here who had no problem getting to the store and changing to device only, while there are those like us who are SOL with these employees who will not budge. The most ridiculous thing I was also told by another employee from a different store was that even if you bought device only, they will require you to open the phone and activate it in the store. That of course makes zero sense
Just reserved an AT&T 16gb gold by creating an att account @ soho. I was planning to exchange my SG ( Tmo GSM version) with the one I reserved. After reading your post, i feel like I won't able to exchange it tomorrow