LH A380 F
I was originally booked on a LH 747 but was lucky to have retained the F seat when they changed the SFO-FRA route to an A380 last summer. It was an amazing flight on row 1 of an A380.
While at FRA LH F lounge I was trying to make a much smaller adjustment to go from economy to biz on a 1 hour LH continuation to Geneva and the answer was they could not change the award ticket (same situation, UAL award ticket).
So my guess is your answer will be no, but it certainly doesn't hurt to ask.
If you get the remodeled 747 LH F, you may actually find it an even better experience with two seats per passenger (one being a permanent bed), however if you get the old style 747 F then it still is a comfortable flight but not at the same level.