The changelogs for stable releases of MPC-HC can be found HERE and for older ones HERE (1.7.13 and below).
The K-Lite Codec Pack usually contains a stable development build of MPC-HC, which basically means a version in between official releases, that contains the latest fixes and improvements. A detailed list of all changes in MPC-HC, including those from development builds, can be found in the source code commit log of MPC-HC.
It is easy to interpret the version of a development build. For example development version 1.9.20.30 equals stable version 1.9.20 plus 30 additional changes after that. To easily find the Git commit that corresponds to a version, look at the About box in the menu of MPC-HC to find the abbreviated Git commit hash.
A full list of changes to LAV Filters can be found HERE.
A full list of changes to madVR can be found HERE.
A full list of changes to GraphStudioNext can be found HERE.
