This post, I think, is mostly useful for me, though it may be of interest to someone ordering a new contract phone that will be swapped to a different number.
I had an iPhone 5 and Emily had a 4S, we're on an AT&T corporate-discount family plan. My i5 was "blessed" by my employer; I can access our Exchange server with it. I ordered a 32GB AT&T 5s from Apple using Emily's contract, but my plan was to switch it to my account, Emily would get the 5, #1 son gets the 4S and his 4 goes to the phone bin . (Somewhat unfair, but the fingerprint activation is very nice when security controls limit unlocked sleep to 5 minutes.)
It may also help to know that we still USB sync to iTunes rather than do iCloud backups -- a retro behavior that will probably go when the iPhone 6 is released.
Some notes on the transfer - which had more than a few glitches:
- The 5s (gold fwiw) came with a SIM w/ Emily's number. So once I activated her other phone was offline. That was a nuisance, it left her without a working phone during the transition.
- I think an Apple shipped phone should be first activated with the SIM (Emily's #) it shipped with. That's what I did, but AT&T activation failed. A message said it was offline. I ended up connecting it to iTunes and doing the activation there.
- In iTunes I decided to first activate as a new phone, then worried the automatic renaming would match my old phone and confuse the backups. Fortunately I'd changed my old phone name from the default.
- During the activation process I entered data, like Location Settings and my iCloud credentials, both by iPHone and iTunes. This is weird; there must be a more standard way. In any case it worked.
- Once the phone was activated I switched SIMs, then confirmed the numbers had switched. I could reach the 5 with Emily's SIM by phone, but not the 5s. (I'll get to that part).
- The full restore required a double-sync, where the first sync only restored config data, and the 2nd restored apps and media. The 2nd sync was held up because the iOS corporate configured security settings demanded a passcode.
- When I restored Emily's 4S backup to her new iPhone 5 I first had to turn off 'Find My Phone'. After restore her passcode didn't work, but my old passcode did. I also ran into the usual odd experience of having to enter iCloud credentials multiple times (icloud vs. me confusion)?
- Emily's AT&T visual voice mail was out of order, it behaved like regular voice mail. Fixing this was an odyssey of its own.
- After the restore Emily had a very large number of apps -- all of my old ones and all of hers.
- To my surprise her iMessage and Facetime services actually worked.
 It needs a new Home button and a new battery; I may pay for a refurb 4 before Apple runs out of them. iPhones have a long useful lifespan and it would be handy to have an unlocked 4 in reserve.