User blog comment:MisterWoodhouse/Pausing UCP migrations to resolve some critical blockers/@comment-45264327-20200901022729/@comment-24155515-20200901041523

Think of it like installing Windows 10 over Windows XP, on the same hard drive. (Ignoring more complex stuff like partitions and virtual machines and such.)

Once you install Windows 10, you can't boot into XP anymore. That's just how it is. Part of UCP is an upgrade from MediaWiki 1.19, the legacy software version, which is many years out of date, to MediaWiki 1.33. You can't go back because they're putting the 1.33 version where 1.19 used to be (and for good reason). 1.33 is the 10 to 1.19's XP.