We're in the process of migrating an old site (v3.x) to v5+. The site has ~20k user accounts and ~900k entities.

When running elgg-cli upgrade, we found that the 3->4 migration takes a very long time due to the Content Owner Subscriptions upgrade. While all of the other 3->4 and 4->5 upgrades take just a couple hours combined, the 3->4 Content Owner Subscriptions upgrade takes 27 hours. This poses a challenge for our migration plan, because we'll have to schedule a large block of time to have our old site offline while the migration occurs.

I think I understand the gist of the upgrade: make sure content owners are subscribed for notifications on every entity they own.

What I'm wondering is whether the Content Owner Subscriptions upgrade is critical and must be performed before we bring up our new v5 site, or is it safe to defer until after we go live with the v5 site? If it's the latter, then we could schedule periodic smaller chunks of downtime over future weekends to run the migration until it is complete.

Our site consists of several custom plugins that manage notifications in various custom ways -- we're really not using any of the core plugins like Groups, Blogs, etc. For us, it wouldn't be the end of the world if not all notifications went out until after the Content Owner Subscriptions upgrade eventually completed.

Thank you!