The high-level checklist:
[X] 1. Merge the first patch for {T167246} (adding new schemas and code)
[X] 1.1. Check deployed extensions for needed updates.
[X] 2. Perform schema change (T188299)
[] Interrupt: Remove read-both ([[https://gerrit.wikimedia.org/r/c/mediawiki/core/+/461440|gerrit:461440]])
[] 3. Turn the feature flag to "write both, read old". See if stuff breaks.
[] 3.1. Announce the pending change to wikitech-l@ and cloud@, and give time for people to update.
[] 3.2. Make sure all deployed extensions are updated.
[] 4. Turn the feature flag to "write both, read new". See if stuff breaks.
[] 5. Run the maintenance script(s) to migrate all the old stuff to new stuff, blanking the old stuff in the process.
[] 6. Turn the feature flag to "new only".
[] 7. Merge the second patch for T167246 (removing old schemas and code)
[] 7.1. Submit schema change (task TBD)