The fault on this one is hard to assign. LH makes a change (sticker upgrade) on a UA ticket and UA determines the ticket is out-of-sync / different than issued. The sticker upgrade is a workaround but the out-of-sync does not recognize -- it is intent for other issues (seat change, time change, flight number change, ....) but cabin change isn't something normally expected. UA can handle UA upgrades UA operated flights. The carriers invented a special system of using miles (and later GPU/PlusPoints upgrades) on other carriers -- so both airlines are aware of what is happening. But cash sticker upgrades are not so supported. It is a practice asking for trouble.