There's a great article on the BBC website today about documents uncovered from the BBC archive relating to regenerations, the first example in particular.
It reveals that Troughton got something of a rough ride from viewers at the time – and that the regeneration process was conceived as some sort of horrifying acid trip.
I've always thought the whole regeneration thing may just have been a back-of-an-envelope kind of thing that could have gone either way.
On several occasions the series has only just escaped cancellation as one of the leading actors takes his final bow, but the transformation between Hartnell and Troughton must have been especially tricky.
It's interesting to posit a world where Doctor Who went the way of Adam Adamant or some other fairly small-fry cult sixties series – and a derisive snort from a big wig at the Beeb could have nixed the resulting 44 years.
There's also some more information on viewers' reactions to new Doctors and press clippings and the like. Seems no-one really gets to grips with the new Doctors at first, though that's a trend that remained throughout all of poor Colin Baker's tenancy, at least until Big Finish rescued Colin and his Doctor.
Much of it presumably derives from the sort of stuff Andrew Pixley used to dig out of the archives, but is worth looking at nonetheless.