Why are you stressed about a 65 min connection? I think there is a very good chance that you will make that connection, in particular if you know how to navigate FRA airport. If you don't make it, you will be rebook on TG921 the following day (subject to seats being available of course) and receive hotel and meals. EUR 600 is also due as compensation unless the delay is caused by extraordinary circumstances. It is tight but most likely you will make the connection.

Just make sure that you don't have important commitments in BKK immidiately after you arrive. LH772 arrives in afternoon so try to keep your schedule free the rest of day to allow for a delay until TG921 arrives in the morning the following day.

Forget about rebooking the ticket - not worth the high costs. There is no way LH will allow you to change the ticket with out change fee + upfare as long as MCT at FRA is complied with.