I do not understand your questions (any of the three of them that seem embedded above).
iOSAndroidRebel wrote:
... can I change to Citizens One ...
Let's try this again. You will pay off your iPhone 6S and end your obligation to your cellular carrier. There would be no "change" involved, as you are now the owner of your iPhone. Barclay is out of the picture. If you want iPhone Upgrade for the iPhone 7, you must pass Citizens One's credit check and set up the upgrade program with them.
iOSAndroidRebel wrote:
... can we upgrade ...
Yes ... if you pay off your iPhone 6S obligation first and end your obligation on your cellular carrier's contract.
iOSAndroidRebel wrote:
... if I ditch, what happens?
"Ditch" what? Again, you must pay off your iPhone 6S and end your existing contract. Does "ditch" mean you are thinking of not paying some money you owe? Then the debt collectors will come after you. This is illegal, and you face fines, imprisonment, or both.