Will giving my fiancée a hand-me-down iPhone lock the phone?

My fiancée has an older iPhone and I'm looking to change out my iPhone X. We're both on the same carrier (Vodafone) and I'm thinking about giving her my iPhone X as we've discussed this and she loves the idea of getting my phone. My question is how can we do this without the phone/SIM locking? I will reset the phone through iTunes and restore her information from a backup, take the SIM from my phone and put it in the new one, and take her SIM and put it in the iPhone X. Will this work?

You don't have to worry about the phone getting locked. As long as it is paid off completely, it should work with the other person's SIM. Simply wipe the phone, take out your SIM, insert your fiancee's SIM and it should accept it with no problems. That's literally all you need to do.