Migration: add a user_id column to the kudos table, indexed but not (yet) unique, unused:
Add a migration as usual, to be used in dev and CI environments.
Add a pt-online-schema-change command in the PR description to be run on staging and beta.
No need to use a foreign key constraint because the kudos should stick around as guest kudos when users are deleted.
No downtime needed because the column is unused.
To test this, a database admin should run the migration (up, down, and back up) and then testers should confirm kudos still work as expected.