aw poor Bernie. however a migrating pin is a much better outcome then another broken bone or something wrong with his hip or knee.
The procedure is easy and quick. He should only have a small incision so the doc can go in and get the pin out. Bernie will be sore for a few days but nothing compared to the first surgery. He'll have a small hole in his bones that will need to heal so your vet will probably tell you to restrict his exercise for the next two weeks. Slow walks and no romping around the house. no beds or furniture, nothing he can jump on or jump off of. He'll be as good as new in a few short weeks and you can be done with this whole ordeal.