I dont know what your experience is or what you claim but if you hand in a carrier unlocked iphone you will get the same in return.
That's how it always works here in the US for years.
The phone is unlocked and same properties get carried on with the replacement when the apple rep inputs the imei in their system.
Why would they replace an carrier unlocked device and give out a unit that is locked to a carrier that you used years ago for example? That doesnt make sense and that's not how it works here.