After an initial hiccup a few weeks ago (request for more miles), I've successfully changed an LH F award three times since either online or via the 1K desk - swiftly and without issues. I've finally found the flights I needed and leaving next week on BOS-FRA-ATH in LH F & C. (Original was LGA-YYZ-MUC). Very excited! (On a related note, the reservation online shows the new rates 110,000 miles but my e-Receipt shows the original 67,500 - should I be worried or is this common?)
I have LH C on the 747-8 for the return home and now just playing the waiting game for something to open up in F. Loads look busy in Sept so far. I've gone from F-->C on this, so I'm nervous about going back to F via a good agent, but according to this thread it can be done...