Yeah,
I did this as well a while back when I upgraded to an iPhone 5 (don't judge me! heh). My pops wanted an iPhone 4S, which I already had, and I wanted a 5 so he upgraded and then we simply activated my iPhone on his account, and then I swapped his over. I think if the SIM card is attached to one number they might just have to charge you 10 bucks for a new one (they mentioned that once to me, but I never paid attention since when I did this I was at the store and they simply setup the SIM card for LTE for my number right away).
Anyhow... works great. I was able to keep my unlimited data, and got an iPhone 5 and he got my 4S happily. Win / Win.
** Before anyone asks... I'm here because I'm switching to an S4 as a company I work with is hooking me up so I can run their new Android build and I'm a big tech nerd so I'm quite antsy waiting **