The fact the Master is shown regenerating retroactively establishes that the decayed Master seen in TV: The Deadly Assassin and The Keeper of Traken is not the same incarnation as that played on TV by Roger Delgado, as had previously been implied in The Deadly Assassin.
The Doctor is from far in the Master's future, and says his age is four billion. Ohila had previously mentioned the Doctor had spent four and a half billion years inside the confession dial. (TV: Hell Bent) However, the Doctor would later describe his age as 2,000. (TV: Smile, et. al)
For the purposes of this list, a "regeneration story" is one in which a regeneration is actually and initially depicted. For this reason The War Games is not included below, even though it is commonly thought of as a "regeneration story". It doesn't actually include a clear scene of regeneration, and the preponderance of stories in other media confirm that the Second Doctor did not regenerate at the end of it. Additionally, immediate post-regeneration stories, like the 2005 Children in Need Special — and ones like Castrovalva, where the regeneration sequence was replayed — are not included.
Regeneration is usually considered a biologic process exclusive to Time Lords and a few other species. However, some consider Regeneration and The Eclipse of the Korven to also be "regeneration stories", though neither describes anything close to a biological process. Korven is a particularly hard one to include in this list, because K9 is said to "regenerate" but is largely physically unchanged.