Rudi, I am learning!
I think you agree that for all sectors besides outbound sectors up to and including first international sector rebooking/revalidation is permitted at no charge. Rerouting/reissue is permitted at a charge of USD 75 per reissue.
I thought that routing change (= rerouting) is eg FRA-HKG instead of FRA-SIN and rebooking is another (*A) flight on the same route.
There are three daily services FRA-SIN with the following flight numbers: LH 9762, LH 9764 & LH 778. So what are the consequences if goodo changes form one LH flight number to the other one assuming D-Class is available?