While working on fixing some of the extensions , we improved the migration guides and created a convenient page listing all of them.
See https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide
The guides list examples of build errors, suggested solutions as well as some explanations and additional references.
Two updates:
The content of all guides is now available on a single page for easy search: https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide (thanks @phcerdan for the help)
There is now a new section called Slicer Extension updates. See https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide/SlicerExtension