yes: must be LH flight number and you must have a 'UA paper upgrade certificate' saying 'good for a one-class-transatlantic upgrade' on LH.

I always got such certificates from UA in advance (without having any flight reservations and/or upgrades made) - time needed to get them sent to Europe was about 3 weeks. They are valid for 12 months - a year ago I had to send one back for recredit 20'000 miles and wasn't charged a fee (I was 1K then).

Those UA upgrades are only good on selected fares (same rules apply as on UA transatlantic flights).