When I updated my SE for a 7 in October, I simply swapped the SIM. The 7 is unlocked and I look forward to being able to move carriers if the need arises.
While unrelated to the OPs question, but still relevant to AT&T, has anyone here had AT&T try to 'increase' your data plan without your consent for $5 extra per month? They did on my plan and I called in to complain that they had no authorization from me to increase my plan details without my explicit consent. I threatened to quit and they reversed the charges and gave me a pitiful credit for my time.
AT&T's position was they were doubling my data per month for 'only' $5 more. I was happy with my existing data plan, had never exceeded my monthly limit and didn't see any need to consume more data whilst having the 'privilege' to pay only $5 per month extra.
pro tip: always buy the unlocked phone, it's always worth the extra cost if buying the phone brand new.
pro tip: always buy refurbished iPhones; they come unlocked.