I'm only mentioning this because I really feel like your current connection is 50/50, at best, but you might also consider the red-eye, SFO-BOS the night before. many, many downsides, not the least of which is a 12 hour layover in BOS, and the loss of sleep. This should not involve any fees whatsoever as the routing/carrier/city-pairs remain unchanged.

Or I'd try to go through IAD or ORD both of which each have two dailies to MUC (on UA and LH). And check that LH direct flight from SFO as award space might open up?