We might not even know come Christmas Day when he makes his official debut.
Consider, When we watched Tennant morph into Smith, he still wore his pinstripe suit and trainers. It wasn't until the end of "The Eleventh Hours" that Matt donned his tweed jacket and bow tie. Of course, intrepid fans with camera phones managed to snap candid photos of Smith in his final ensemble a few months before the episode broadcast.
If Capaldi's initial series doesn't air until the Fall of 2014, we probably won't know his outfit until next summer, a good nine months from now.