I actually had the option of upgrading J-F on SFO-FRA last December for 500$, fixed fee on LH website. I was tempted to have my first F experience, but ultimately didn't take it as I just didn't see the benefit on an eastbound flight, 80% of which I slept through anyway. So I agree with 59Impala , it needs to make sense. On a daytime flight, with time to spend at the FCT, sure.