[JenrHywy]

We have a similar sounding app in terms of scale. We just bit the bullet and a couple of us spent a few weeks doing the upgrade in one branch while the rest of the team worked in another. We'd periodically merge in to the upgrade branch and tweak things as needed.

It wasn't too bad, really. Mostly it was updating any new controller methods to use the new attributes.

We originally planned to upgrade incrementally (one project at a time) but to be honest, it looked that was going to be far more fiddly and annoying that just doing the full thing.