A great strength of a megadungeon is having a single location where delvers can go and effect changes on the environment. These changes can be temporary - unlocking doors, disarming traps, "clearing" a high-traffic room - but they are often going to be persistent - monsters dead, walls knocked down, doors removed. It's a combination of the immediate and the cumulative. The delvers that survive also change temporarily (injuries, fatigue, etc.) and permanently (gained experience, found gear, death).
So the PCs level up.
I asked back in 2013 if anyone had the monsters leveled up, too.
Clearly, Gary Gygax felt that way:
Gygax on Leveling Up Monsters
So it's not a new idea. Some monsters get more dangerous as they repel attacks just as the PCs do. The question is, do they gain experience from fighting and killing delvers? Or do they gain experience from looting the fallen, much as the PCs? Or purely ad hoc like Gary Gygax did? So many good choices . . .