I believe the last chance of upgrade will be to pay immediately after boarding for the higher cabin. The only other occasions passengers may move to upper cabins during flight are:
* Before landing if disembarking with unusual circumstances (I was moved to the F cabin to disembark with my mother-in-law as she needed special attention);
* Medical emergencies or related;
* Seat malfunction with no available seat in same cabin (in which case they may still ask a DM to move to a higher cabin to leave you a seat in the same cabin).