Overt Narc morphing into Covert?

Interestingly, my NH was a covert narc in the earlier years of our relationship, but has morphed into a covert narc. (Married 26 years). I’ve often wondered if it was a natural progression of NPD, had to do with aging, or had to do with how I interacted with him/responded to him.

Has anyone else had a similar experience or know why this might have happened? Is it normal, common, etc.?