The regeneration between Jon Pertwee and Tom Baker is, I'm afraid, rather disappointing. These days we are used to volcanic, explosive transformations. Here, we get a straightforward "roll back and mix" shot, as used most commonly for TARDIS arrivals and departures. The Doctor's collar noticeably fails to match between the two actors.
A couple of clever clogs have experimented with new overlaid FX over the years - the results to be found on YouTube. Here's a couple of nice examples.