Yes thanks, I checked in that thread, there is one poster in there who specifically said that they usually were able to do BKK->FRA C->F. However that's a little old now and Lufthansa changed the # of seats in F from 16 to 8, so not sure if it's still true or not.
As far as I can tell, the paper SWU should work. Reading what it says on the SWU, it says good for upgrading Lufthansa business or economy to next class of service, excluding the following booking codes, and Z is not among the excluded codes. So... should work.
Get more miles with LH too, that's a good point. But still basically I think I want the more comfortable seat with Turkish Air, unless there's a good chance of getting F with Lufthansa... then I would want that!!!!