Improvements to one's own theorems What are some notable (famous?) instances where the following has occurred.
A particular author proves:
Every P  which satisfies Q has property Z.
A few years later (roughly speaking)  the same author proves:
Every P has property Z;
thus rendering at least part of their original research article obsolete.
Is such a thing common?
On one hand, the author should be the person most equipped to strengthen their result since they likely tried before, learned some traps to avoid, and got a partial solution.
On the other hand, the author could prefer not to revisit their work to avoid duplication & making their own work obsolete, or simply to do research on different things they find interesting.  Maybe they eventually just "quarantine" the problem in order to make better use of their time and mental energy.
On a personal level, I really struggle with "letting a problem go", especially if it's one I've previously solved certain cases of.
 A: Something along these lines happened during the march towards the odd order theorem. Quoting Wikipedia:

The attack on Burnside's conjecture was started by Michio Suzuki (1957), who studied CA groups; these are groups such that the Centralizer of every non-trivial element is Abelian. In a pioneering paper he showed that all CA groups of odd order are solvable. (He later classified all the simple CA groups, and more generally all simple groups such that the centralizer of any involution has a normal 2-Sylow subgroup, finding an overlooked family of simple groups of Lie type in the process, that are now called Suzuki groups.)
Feit, Marshall Hall, and Thompson (1960) extended Suzuki's work to the family of CN groups; these are groups such that the Centralizer of every non-trivial element is Nilpotent. They showed that every CN group of odd order is solvable. Their proof is similar to Suzuki's proof. It was about 17 pages long, which at the time was thought to be very long for a proof in group theory.
The Feit–Thompson theorem can be thought of as the next step in this process: they show that there is no non-cyclic simple group of odd order such that every proper subgroup is solvable. This proves that every finite group of odd order is solvable, as a minimal counterexample must be a simple group such that every proper subgroup is solvable. Although the proof follows the same general outline as the CA theorem and the CN theorem, the details are vastly more complicated. The final paper is 255 pages long.

