I would advance individuals arbitrarily as needed, but generally lagging behind PCs. New 'higher power' NPCs can be introduced when necessary, as the characters move on and upward.
I also use a simple mechanic for 'graduating' npc classes into 'real' classes when necessary. When I desire to, when an NPC class I will exchange one of their lower levels for an additional standard level...
For instance: An NPC who is Adept 4/Expert 5 is playing a more pivotal role in the game. While making this transition, the NPC gains enough for another level (or arbitrarily), and actual Wizard levels are desired. The Adept levels are then intended to be upgraded to actual wizard levels. The NPC becomes Wiz 2/Adept 3/Expert 5 (gaining 2 wizard levels and dropping one of the adept levels). When the next level is attained, the NPC becomes Wizard 4/Adept 2/Expert 5.
While this can create weird situations of power level drop in the area of spellcasting, it does allow for a more orderly transition, and the power drop is corrected quickly by the rapid advancement in the 'true character class'.
Although I do not have a formal system for which npc classes upgrade into what pc class, I tend to do this in an orderly way. I also use a number of other additional NPC classes in my game to fill certain roles. My example above would actually work differently as I do not use adepts as written, but Priests and HedgeWizards/Witches. Names for these extra NPC classes mean little... one might be called a Witch, while another might be called Alchemist, while another Witch-doctor (this applies for all the npc classes to keep players guessing).
Warriors for Fighters
Hunter to Rangers, Scout (hunter = Ranger-lite)
Tribesman/Clansman to Barbarian
Squire/Aristocrat to Paladin, Knight, etc.
Priest to Cleric, Favored Soul, etc.
Witch/Hedgewizard/other to Wizard, Sorcerer, etc.
Expert to Rogues
There are others. In most situations the warrior is the base for the other NPC classes associated with fighting, with just minor changes (skills, minor abilities) to make them a little more suited for the role they take. Minor Knights or similar style mounted warrriors are simply Squires (the class) which are just warriors with some skill changes.
I think that is enough info on my own system of NPC classes and their advancement to get my concept across, eh?