More to the point of asking the question: Should racial and religious heroes available early in the game be able to upgrade into more advanced versions of themselves after acquiring later-game techs? Is the early utility of these heroes balanced by their potential limited usefulness late in the game?
For example: Gilden Silveric is one of the earliest heroes available in the game. When you get him, he's actually fairly awesome, because he's a kick-ass Archer. But that's all he'll ever really be, because he can't develop additional core abilities as the game progresses, only gain promotions available to a hero Archer. A Longbowman is able to use Bronze and Iron weapon promotions, never mind the much higher base strength of a Flurry or Marksman unit.
For example: Gilden Silveric is one of the earliest heroes available in the game. When you get him, he's actually fairly awesome, because he's a kick-ass Archer. But that's all he'll ever really be, because he can't develop additional core abilities as the game progresses, only gain promotions available to a hero Archer. A Longbowman is able to use Bronze and Iron weapon promotions, never mind the much higher base strength of a Flurry or Marksman unit.