Are audiences really going to enjoy watching a Han Solo that is not Harrison Ford? Think about it. What is it about Han Solo that you like that wasn't actually just that you like Harrison Ford? It's like trying to have an Indiana Jones that's not Harrison.
Obi Wan only worked for Ewan McGregor because Alec Guinness was such an old character.
But an entire film about a young Han Solo? Ford was like 30 and looked 22 when he was Han Solo? He made the character.
Granted it won't matter anyway as the sheep will line up to see anything Star Wars related.