Do to a booking error (my fault) through a travel agent, I have an LH P class fare booked on the first 2 segments of a 6 segment itinerary. I would like to change the P fare from the second segment (FRA to DXB) to a Z fare for a possible upgrade to F. The rest of the segments are Z fares.

The question is: do I need to do this in advance of the whole itinerary starting, or can I change the second segment to a Z fare after the first segment has been flown?

Thanks in advance.