I booked a flight to Puerto Rico using a combination of a travel voucher (received when volunteering to be bumped) and United quest credit card. Today I noticed that the flight is about $200 cheaper. Is there a way to safely rebook for the lower rate? I tried changing the flight to the existing flight, but it won't let me. Would I be safe changing the flight to another flight and then changing it right back? I just want to make sure I don't actually lose my current flight.