For adding new apex enemies to old zones, maybe you could add them in a layered way like you have for enemies in Domain Invasion or Gaes Fete. A NPC at the entrence of the zone could give players a buff to access the Apex enemies. This would allow you to scatter Apex enemies anywhere in any zone without it harming lower level players.

Alternatively you could add them to old content like Moblin Maze Mongers. Adding a new marble that sets enemies to apex level probably wouldn't be to hard, and it would revitalize some old content.