Originally Posted by
Duke787
Across the board (DL, AA, UA) the biggest issue is typically just that there can be a fee for the airline taking ownership of the reservation which is needed to process instrument / mileage upgrades
Delta does not charge to take over a third party booking if you are using upgrade instruments. They also don't charge the fee when you call in to do a same day change. If you have paid a fee, you should not have.