The first Doctor had a hard time fighting the Cybermen in The Tenth Planet. He emerged triumphant but physically and mentally exhausted and in front of his astonished travelling companions, Ben and Polly, collapsed on the floor and began to transform into a younger – well younger-ish - version of himself. The Second Doctor emerges.

The Doctor Programmes their Escape Route From The Tenth Planet His Eyes Tell of The Weariness of Battle Something Feels Wrong This Old Body Of Mine Is Wearing Our I Must Get A New One Polly anxiously bends over the collapsed form of The Doctor The Transformation Begins Halfway There A New Face Oh! Oooh! Oh Dear!

The First Time We Saw A Regeneration - October 29th, 1966.