Can we exchange 3G & 3Gs phones and switch SIM cards?

[ODavy]

I have a 3G and my sister wants it when I get my 3Gs.
Problem is that I'm not eligible for the cheap 3GS iPhone till December.
Sister doesn't have ATT so should be eligible for cheap 3GS iPhone but is broke and wants my FREE 3G.
If she buys the 3GS and, I suppose activates it, can we exchange phones and switch SIM cards?
Any info apprecialted

[Dizzy714]

To me it seems like it would be fine, since the plans are exactly the same. My situation might be a little different though. Because I'm going to try using another number on my line to buy the cheap 3Gs, then pop my sim card in and take over - I currently have a 3G right now, but I still think I'm going to have to call AT&T and have them tinker with the plan after I get the new phone.

[CaptainChaos]

Give her the money for the 3GS, let her sign up with her own account, and then swap phones and everyone wins...

[ODavy]

Thanks - that's what we'll do.