I took the GAS tour last year and flew into Frankfurt and took the two trains ( about 3 Hours) to Trier. Rick Steves will give you details on how to get to Trier from the Frankfurt airport. The train station is in the Frankfurt airport. Then fly home from Vienna. It is called and open jaw ticket and easy to purchase. I would suggest you change your ticket, but in doing so there will be a fee and it can be up to $300.00 depending on the airline. It may be less like $100 or $150.00, just call and find out. You may also fly on two different airlines, no big deal but depending on the one airline they set the price of the exchange fee. I still would fly into Trier at least one day in advance if not more if you have the time. Trier is a lovely city and worth seeing before the tour.
The tour is excellent and you will have a wonderful time.