Per the parent ticket, we need to a migration path to avoid breaking rendering of existing translatable pages. In order to do that, we need to:
# track and store syntax version of a translatable page
# allow migration when re-marking the translatable page
# (deferrable) provide a list of unmigrated pages
== QA plan
Affected projects: All Wikimedia projects using Translate and translatewiki.net
Pre/post-deployment QA: Check Special:PageTranslation, upgrade a page, create a new page and mark it. Neither should be using the old syntax. Check that form elements are consistent (if OOUI patch is merged by the time)
== Expected outcome
New pages will use new syntax version, old pages can be migrated by translation admins when marking a page and ticking a checkbox.