It has been reported that Arsene Wenger didn't want Upson to leave, which would indicate that the manage felt that he did have what it takes to succeed. However it is clear that Upson didn't want to wait around for his chance, and in some ways I can understand that he wants regular first team football.
He had a spell out on loan at Reading earlier in the season, and upon his return obviously made it clear that if he couldn't have regular first team football he'd want to look for another club.
I don't feel that it is a sign of lack of ambition on the players part as has been mooted in some quarters - in some ways one can't blame him for wanting regular football, and if he feels that Birmingham is the right move for him then who are we to argue?
I wish him all the best with his new club.