Apple replaces like for like. If it was locked, the replacement will be locked.
That said, if when you turn the phone on you are presented with a screen that allows you to choose your carrier then this is your opportunity to choose a carrier whose policy is to unlock their phones.
That's the experience I had in March when I turned in a Sprint iPhone 5 locked to Sprint. Because I was presented with that screen I was able to choose Verizon (which unlocks their LTE enabled devices) and the phone unlocked.
If you are NOT presented with that screen then you can be pretty sure it's still locked to your carrier.