Lufthansa 646, code-shared as UA9116 has three classes of service: Economy, Business and First, with "J" class getting you a Business-class ticket and "F" for First.

Business Class is sold out, but they will waitlist me. I *think* I can pay for a J but be assigned an Economy seat. I'm going go try this now.

So then that will get me to booking as a UA9116-J and then I'd like to use an Upgrade Certificate to get to UA9116-F.

Which was the original question.... But it is a bit more tricky. At least online, UA wont offer me UA9116-F unless I book IAH->IAD->FRA->TSE (in which case 4 seats are available on the last leg). If I book it as IAH->FRA->TSE then it denies the existence of "F" class, and says only that Business Class is sold out, so I have to take Economy.

LH646 shows availabilty in FirstClass no matter how I get to Frankfurt, so I can also book it UA to FRA on J and LH to TSE on F. Hm... well, that would be if my company allowed me to book "F" class tickets.

If I book as LH, then upgrade awards appear to be valid but only on day of travel.

AAAGH... :-)