I might be missing something but the combination of 'we mustn't break anything' and 'we can't test it without 2-3* team size' sounds like release deadlock until you can test it..
The migrations where I've worked at have always been a normal ticket/epic. You plan it in the release, you do the migration, you do the other features planned, do the system tests, fix everything broken, retest, fix, repeat until OK, release.
Otherwise you're hoping you know exactly how things interact and what can possibly have broken, and I doubt anyone knows that. Everyone's broken things at first sight seemingly completely unrelated to their changes at some point. Especially in large systems it happens constantly. Probably above 1% of our merges break the nightly in unexpected places since no one has the entire system in their head.
Or you're keeping a dead product just barely alive via surgical precision and a lot of prayers that the surgeon remains faultless prior to every release.
The migrations where I've worked at have always been a normal ticket/epic. You plan it in the release, you do the migration, you do the other features planned, do the system tests, fix everything broken, retest, fix, repeat until OK, release.
Otherwise you're hoping you know exactly how things interact and what can possibly have broken, and I doubt anyone knows that. Everyone's broken things at first sight seemingly completely unrelated to their changes at some point. Especially in large systems it happens constantly. Probably above 1% of our merges break the nightly in unexpected places since no one has the entire system in their head.
Or you're keeping a dead product just barely alive via surgical precision and a lot of prayers that the surgeon remains faultless prior to every release.