Migrating primary keys from int to bigint is feasible. Requires some preparation and custom code, but zero downtime.

I'm managing a big migration following mostly this recipe, with a few tweaks: http://zemanta.github.io/2021/08/25/column-migration-from-in...

FKs, indexes and constraints in general make the process more difficult, but possible. The data migration took some hours in my case, but no need to be fast.

AFAIK GitLab has tooling to run tasks after upgrade to make it work anywhere in a version upgrade.