Since PHP 5.1.0, every such release has included a migration appendix / upgrade guide. This involves the following:
Currently the internals team has created the README files, but this may change.
Some extensions may have been moved to PECL, became bundled, were re-categorized, etc. This information should be updated at this time, or at any time before:
NOTE: This information comes from the comments found within (on top of) every book.xml. Status, and Membership. The Purpose dictates how the extensions are categorized in the index/table of contents.