There's a bit of a history of recycling actors from minor / cameo roles, into more leading roles... playing a new character... or even the Doctor...

...Colin Baker was in Arc of Infinity, when Peter Davidson was the Doctor, etc...

It was Bonnie Langford that really scuppered Sylvester McCoy's stint, for me...

I quite liked a lot about Colin Baker... just unfortunate I thought, that his tenure didn't really work out, for all sorts of reasons outside of his control as an actor, I think.

It seemed like everyone from the wardrobe department up to the controller of BBC1 had it in for him ;-)