An adventure...
Flew CDG-FRA-IAD, UA-designated. Called MP to upgrade FRA-IAD, was transferred to another agent (not sure where). She seemed to know the process, told me she had to call LH, then rebook and reissue. About 25 mins on the phone, all done, upgrade deducted and I got a confirmation email (with a ticket number, but no PNR).
But when I looked at my res, that segment was missing. Thought it might have been on another PNR, but wasn't showing in my reservations. Of course can't search by ticket #. Called LH for a seat, after lots of searching they said they found it, but ticket was mismatched. Called UA again, they said everything was fine. Of course I knew it wasn't, but didn't have more time to chat on phone so I printed out the confirm and hoped for the best.
CDG is now outsourced, they said they didn't see the segment and I should deal with it in FRA. FRA immigration let me through with itinerary, so I headed to Senator. Agent there didn't see a res at all, sent me to biz lounge, where there's a UA agent sitting with LH one. (Who knew?)
Anyway, long story short -- the LH agent was excellent, completely took ownership ("don't worry, we will fix this"). Many interagent discussions and several phone calls later, ticket was reissued and I got my BP.
Without a very long connection and a really determined agent, not sure what would have happened. A warning...