What matters is your account status on the date the individual flights are flown.

Flights are always considered individually.

How long a gap do you have between the flight that will get you status, and the next flight after? It takes the system a day or two to do the upgrade.

EDITED TO ADD: Backdated flights can get messy, i.e. where the flight date was before a yearend/upgrade, etc. It would complicate things to cover every permutation here.