When I bought my iPhone 6s from an Apple Store, I paid in full via credit card for it. Wound up with buyers remorse overnight, so called them up the following day and asked if I could change to going back on contract via my carrier with them. Yes they said, come into the store. Went in later that day; they checked the phone back in and refunded my credit card. Then took out a carrier contract through them with my existing network carrier and paid the smaller up-front fee.
I’d say as long as you’re civil & polite about the matter, they’ll likely be accommodating with you. I’ve also seen people starting with demands and threats in store and seen them get nowhere.
