The Doctor In The TARDIS

An original draft of the script for The Tenth Planet has recently been unearthed and along with various differences to the transmitted version of the story, this draft features an ending in which the Doctor does not regenerate, but continues on to his next adventure, still played by William Hartnell.  The draft, which was found by author Michael Seely along with draft scripts of The Moonbase (then known as The Return of the Cybermen) in the attic of one of Kit Pedlar's children, was dated to June 1966.  It had been prepared by Gerry Davis during a period when Kit Pedlar was ill and, most notably, was written the month before the decision that Hartnell would leave was finally made.

Seeley had this to say about the script:

The structure is more or less the same, though a lot of the dialogue is different. Some things were cut, especially involving the Cybermen. For example, the Cybermen planned to convert Polly and the Doctor into Cybermen towards the end of the story, and kept them prisoner in what they described as a waiting room. The most eye catching difference is what didn't happen at the end of the episode.

It sounds like there were some interesting changes even before the major one - the regeneration - which would go on to influence the future and long-term success of the show in ways no one could have foreseen that summer in 1966. As always, pieces of evidence like this give us exciting and intriguing glimpses into how the show might have been, had circumstances been different.