Equipment Change and P+ or E+

I did some searching on the sub and looking for validation.

Have booked a flight that was originally on 787-9 and now the equipment was changed because schedule was very slightly changed? (within 40 mins of original time.) Both flights were 787-9 change to 777-200. Question is if we bump ourselves up to E+ or P+, and they change equipment again will it maintain these upgrades? I believe it should. Id rather have a few more inches available on the 777-200 if possible with E+ and P+ is still $800+.

For example, bulkhead row is available and would be convenient for our group to be seated together.

Thanks!