A lot of things only make partial sense here. LH native flight number conversions are free ... sounds like you needed to upfare to get to an UG eligible fare class? PE seat assignments on LH are not free and they are not refundable either (unless there is involuntary change or cancellation). If you make a change to the ticket it will get re-issued which will likely mean the seat assignment on LH is gone; LH does transfer seat fees to new flights, but you will likely need to call and it'll probably be difficult since these are not 220 tickets. Should be solvable though ... https://www.lufthansa.com/ge/en/seat-reservation