Originally Posted by Loki882
Not necessarily. We never see Boba's face in the OT. The switch could have happened in between Episode's III and IV, and it wouldn't be a contradiction. The guy in the OT was simply using the Boba Fett name.
The problem you'd likely run into though would be the question of why this other person would want to use Boba Fett's name. Off the top of my head, I can only think of two situations, really - either the guy is some random, no-name schmuck who is trying to benefit off Boba Fett's reputation rather than work to earn it himself (which would undermine the character we see in the OT) or it's someone who is trying to conceal his identity (in which case, it's his original identity that is more important than the "Boba Fett" one).
Secondly, it makes the situation needlessly convoluted for little to no benefit.
As such, I think a better idea for a "Boba Fett" spinoff would be to simply make a "Bounty Hunters" TV series. You could set it either shortly after RotJ or around the time of the new trilogy; with the premise of the main characters (including Boba Fett) hunting down Imperial war criminals for the victorious Rebel Alliance.
This way, even if an identity switch is felt to have been necessary to further develop the character, the longer run time provides us more time to grow attached to the character prior to the reveal while the ensemble cast would de-emphasise the importance of the "Boba Fett" identity/name compared to a self-titled movie. Alternatively, the post-RotJ timeline allows Temuera Morrison to play the character.
Plus, in terms of appeal, you'd attract Star Wars fans. You'd also almost guaranteed to attract Firefly fans due to its similar premise. Plus, the general audience who'd likely give the show a shot due to the brand name.