When normally upgrading a phone you do not usually have to hand back the old one. I am with EE and my last upgrade was from iphone5 to iphone6 Plus. After paying a small upgrade fee for the phone it was quickly despatched to me. The iphone 5 went to my wife.
I am due an upgrade in June 2017, so I will be going for the iphone 7, and I do not expect to have to hand the iphone 6 back.
One word of warning re iphones, if you get a cracked screen get it repaired by Apple otherwise they will not touch if it if it has been repaired by an unauthorised dealer. Happened to me, cracked the screen got it repaired, had another fault, went to Apple and they referred me back to the original repairer despite being still in warranty, which was of course invalidated as is case with most companies. Lesson learned.