Originally the Doctor was supposed to regenerate in the final episode of Series 10, with the series picking up at the beginning of Series 11 - with no Christmas special. But because this could have meant they would permanently lose the Christmas day slot, they decided to have one more episode with the 12th Doctor. This video shows how the finale could have ended if it featured the Capaldi - Whittaker regeneration.
It's also now with 100% more Rory. Also, the 5th Doctor had a different hair colour straight after regeneration, so why can't the 13th? :P
