Upgrading/updating from old WPMU install

There’s plenty of info on how to upgrade older versions of WP, but I’m not finding much info regarding how to make an old WPMU install current. I inherited and ported a legacy install of WPMU 2.8.6 and obviously want to get it more current to take advantage of security and features presently available.

I see from this: https://codex.www.ads-software.com/Create_A_Network that auto-upgrading from WPMU ended as of 3.1. Which makes me wonder if the simplest would be to manually upgrade to 3.0 and then auto-upgrade. But in any case, with a WPMU install, I don’t know how much I have to deviate from the normal upgrade path.