This is an RFC to migrate code-review from Gerrit to Phabricator's Differential.
Full RFC on mediawiki.org:
https://www.mediawiki.org/wiki/Requests_for_comment/Migrate_code_review_and_management_to_Phabricator_from_Gerrit
Plan as it relates to WMF RelEng's quarterly goals:
https://www.mediawiki.org/wiki/Wikimedia_Release_Engineering_Team/Project/Differential_Migration
(just a team/goal planning document, but it helps get an idea of the steps/overall plan)