Checking my own sanity here:

Current reservation is a TK metal award from TBS-IST-SFO booked on promotional business. Looking to change that to MUC-IST-SFO, with the new first leg departing a day earlier, resulting in an overnight IST connection that is still <24hrs so not a stopover.

The MUS-IST new leg has promotional award avail, but the IST-SFO does not anymore. This is an allowable change to make, just a $25 fee, correct?

I've tried the call center 3 times with 3 slightly different answers. Twice that it's not possible to change just one leg, and a third time the agent started to look but couldn't see any of the same MUC-IST availability I was seeing on the miles & smiles website. Going to try contacting a sales office via email next, if this should be a valid routing change.