transatlantic upgrades for LH flights using Mileage Plus paper upgrade vouchers can be done:
* by UA agents only (not LH)
* only if flight number and carrier is LH (no UA code-share flight-number), unfortunately on such bookings (LH flight number and LH=carrier) you don't earn UA Mileage Plus status bonus (no 25% Premier or 100% Premier Executive / 1K status bonus then)
* if space is available, ruled by UA space-'policy' /upgrade availability for that specific LH flight (LH might have no more upgradable seats in the inventory, and UA might still have - or the other way round ...).
* on selected fares only (the same fare rules apply, more or less) as for upgrades on UA transatlantic flights)
* you must ask for the Mileage Plus paper upgrade voucher, good for a LH transatlantic flight, in advance (calculate about 3 weeks until delivered) from Mileage Plus (costs 20'000 miles for one-way) (that paper certificate is good for 12 months from date of issue.
* the paper certificate is transferable (but nor for sale or bartering)