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)
[X] Interrupt: Remove read-both ([[https://gerrit.wikimedia.org/r/c/mediawiki/core/+/461440|gerrit:461440]])
[X] 3. Turn the feature flag to "write both, read old". See if stuff breaks.
[x] 4. Run the maintenance script(s) to migrate all the old stuff to new stuff.
[x] s1
[x] s2
[x] s3
[x] s4
[x] s5
[x] s6
[x] s7
[x] s8
[x] wikitech
[ ] 5. Turn the feature flag to "write both, read new". See if stuff breaks.
[ ] 5.1. Announce the pending change to wikitech-l@ and cloud@, and give time for people to update.
[ ] 5.2. Make sure all deployed extensions are updated.
[ ] 6. Turn the feature flag to "new only".
[ ] 7. Remove old schemas and code
[ ] 7.1. Update WMCS replicas to no longer reference old schemas.
[ ] 7.2. Write and merge patches to remove `$wgActorTableSchemaMigrationStage`, supporting code, and old schemas.
[ ] 7.3. Submit #schema-change task for WMF production.