Info Text from the Tenth Planet.
‘The change of Doctor Who’s lead actor was the first time such a recasting was acknowledged and explained as an overt part of a series’ ongoing fictional narrative. Ideas for transforming the Doctor were discussed between Innes Lloyd and his team. The basic premise was that he should 'die’ and come back as a new, younger man. This echoed an idea in the 1963 novel A Trace of Memory by Keith Laumer, in which an alien marooned on Earth takes on a rejuvenated form after slipping into a coma and incurring memory loss.
Innes Lloyd himself was more influenced by Doctor Who’s time travel aspect. He reasoned that, if the Doctor could travel in time, he should also be able to change his form and personality. The thinking was that we all are different people at different stages of our lives. The idea had been explored in Peter Ustinov’s play the Banbury Nose (1944) , in which a series of different actors played the same character at successively younger ages, moving back through the generations of one family. The idea thus crystallised that the new version of the Doctor should be a younger persona than the original, but still the same man inside.
… At first they assumed it would be technically impossible for the physical change between Doctors to be depicted on screen. They envisaged that the old Doctor would simply collapse to the floor with a cloak covering his face, and the new Doctor would not be revealed until the next story. Their plans changed when they learned that a suitable effect could be achieved. Vision mixer Shirley Coward would cross-fade from a close up of William Hartnell’s face taken by one camera, to an identically composed close-up of Patrick Troughton’s face taken by another.
On the day, Shirley Coward in discussion with Derek Martinus and others, devised a way of enhancing the effect. BBC mixing desks at the time had two banks of faders- Bank A and Bank B. In Riverside studio 1 however, Bank B had a known fault and was not supposed to be used. Coward mixed from the close-up of Hartnell’s face on Bank A to the same shot on Bank B. The fault then caused the picture to flare and distort. Coward next faded down that flaring shot and simultaneously faded up the close-up of Troughton also on Bank B. Finally she mixed from that shot on Bank B to the identical shot on Bank A, causing the flare to fade away.
As originally scripted the story’s last words were the Doctor’s 'No, no, I can’t go through with it. I can’t, I can’t. I will not give in.’ In the end Derek Martinus chose to stage the crucial sequence with the Doctor silent, conveying the action in rapid intercut shots.’