You can keep or plan. The only way you would have to change your plan would be if the
phone was part of a promotional offer that required a plan change.
gramps28 is correct. I just upgraded my phone and was not required to change my plan.
In fact, the phone I wanted, the G6, was on discount but only with the qualified, post paid plan. Which I did not have since I am not a fan of paying for things I do not need (unlimited data, Tuesday deals, etc.).
I was at the store for about 20 minutes, manager heard I wanted the phone and was ready to pay the discounted price. He agreed and I walked out paying almost $200 less then retail.