With the whole Adam/He-Man thing, I'd prefer two actors who look kind of alike. He's got the secret identity, so as far as most of Eternia is concerned, they're two different guys.
But anyway, I suppose it's kind of like Iron Man. You've got Tony Stark who gets the character development; when he's kicking ass as Iron Man we can't really see his face (except in close-up), but we're still rooting for him because we know it's Tony in there. And by the time they did Avengers, RDJ almost never wore the suit.
Similarly with Adam, he would go through the whole hero's journey in getting the sword and fighting Skeletor; we would spend enough time with Adam in the first part for us to understand his plight and root for him to win that by the time he's replaced (physically) with He-Man, he's built up enough as a character that we can continue rooting for him even though he looks different.
Am I making any sense here?