SEs have priority over Es even if they are put on the wait list after an E (i.e. between 2 and 4 days). The waitlist will likely be visited by load controlled just prior to the P 2-day window opening, and upgrade inventory could be released. If that occurs, all SEs on the list will clear first, then Es based on when they were put on the list. Any remaining seats would be available for Ps to take when their window opens. If the wait list remains through the P upgrade period, but upgrade inventory is released the day before the flight, the priority will still be SEs, Es and then Ps, all based on when they were put on the list.

However, if the list does not clear prior to flight, on the day of flight it all starts again from scratch. The new waitlist will be based on when elites checked in and asked to be put on the new list. Priority will go to revenue passengers and those holding W space on D award tickets, based on status. Then cert upgrades will go to SEs, based on when they checked in, then Es, then Ps.

BTW what does the back cabin look like. They may not be releasing seats in the front if the flight is overbooked in back, and if that's the case, you might get an opup when you check-in. These too are processed by status, but are generally preprocessed when a flight is overbooked and noshows are not expected. That list is prepared by status, irrespective of when one checks in, as it is all done hours before the flight is to depart.

If you provide more details of route, date, etc. we might be able to provide a better idea of how the flight is likley to clear upgrades.