Thanks for all the input and suggestions, everyone!

Eventually got passed to a supervisor who approved the change. Her original argument was "the operating carriers must remain the same, so any rerouting must remain operated by UA and LH" (an additional wrinkle is that the original YYC-FRA segment was operated by AC but marketed by LH). After I made her aware that the original itinerary was in fact operated by UA (SFO-YYC), AC (YYC-FRA), and LH (FRA-BEG), she changed tune to "the marketing carriers must remain the same". In this case, I pointed out, the revised itinerary I suggested could be booked entirely through UA-marketed flights. Not sure if any of that was persuasive, but she eventually agreed to make the change and waive all additional collection.

To answer those wondering why I didn't want to fly SFO-FRA (or SFO-MUC) direct: I wanted to keep the TATL leg operated by AC to earn some AQDs (AC's equivalent of PQDs).

Total call time: 1 hour 18 minutes. But at least I'm satisfied with the outcome.