The price can vary after OLCI opens but often doesn't - though of course the option to upgrade may disappear entirely. Unlike many US airlines, European ones are perfectly content to have planes go with some empty seats in J/P so AF will ask for what it wants for the seat rather than try to fill them at all cost or "bargain" on what has not been filled.

Also 4 unallocated seats does not in any way mean that there are four empty seats! There may well be J pax who are confirmed with no pre-assigned seats and who have not checked in yet, especially when you have a connection before the flight you want to upgrade.