Changing the IMEI of iPhone

My Apple products are recycled down to sister, niece and nephew, family back home. Bear in mind that this is in a foreign country, third world.

I sent my sister an unlocked iPhone that I had bought new, unlocked and used in the states with AT&T and tmobile. She had been using it fine for over a year when
I sent her another new, "old one". She handed down her good, working phone to my nephew. He took the iPhone to a local store to get the sim card cut to size. There they told him to come back the next day to pick the phone up.

When he received the iPhone back it didn't work anymore. The iPhone was locked.

My question is can they just take the whole inside of a locked or blacklisted iPhone and replace it with the original shell of an unlocked one? That would be the only answer to me because they had to have changed the IMEI from a good unlocked one to a non working IMEI.

I can see why they would do this. They probably buy very cheap unlocked or blacklisted iPhones and switch the innards from good ones and customers that don't know better are told, like my nephew, that it locked itself when he reset it the iPhone.

And no, my sister doesn't have the original box nor a copy of the original IMEI as proof, and neither do I. I do began to take a picture of the about page of all iPhones i sent them now.