Doing a cross upgrade is QUITE common...
When we got my wife's OG Evo at best buy, we used the upgrade from another line, and the best buy rep did the switching of the Evo to the proper line AND the restoreing of the original phone to the line whos upgrade we used. Super super common.
Just yesterday, we got her an LTE and again used another lines upgrade to do so, but this was at walmart, and the rep there claimed they could not do the switch (even though that was a total lie, it was just the rep being either too lazy or not knowing how to do so).
So immediatly after paying for the phone, before even leaving the store, I called sprint customer care, said "I just upgraded line X, but it was supposed to be a cross upgrade and the walmart rep said they couldnt do the switch for us. So I need the original phone put back on line X, and the Evo LTE put onto my wifes Line Y" the rep said "no problem" and within 5mins and a single restart on the Evo LTE, all was said and done and both the upgrade lines original phone and my wifes new Evo LTE were working 100% as they should.
Definatly NO need to wait any time at all before calling and doing the swap (Certainly not a couple days). And like I and the guy a few post above me said, at best buy, they will do the whole upgrade and line swap for you 100% no problem. I have done that almost a half dozen times, never had a problem, always went smooth and was done in 15-20mins.
But the only place we could find a phone for her locally was Walmart, so thats why we went there yesterday, and yeah, flat out, It was either the rep not knowing how, or being lazy and not caring to do it for me.
Both Best Buy and walmart (and all 3rd parties) use the same program to work with Sprint accounts (program called BEAST) so the walmart rep definatly had the PHYSICAL ability, but just didn't...