There's a way to save your already-installed extension, in "Manage Extensions..." Enable dev mode, then Pack Extension.
However the browser will probably just refuse to run it soon.
Vivaldi, for what it's worth, seems to still run uBlock Origin just fine. I am afraid to uninstall it now to test if it'll re-install properly.
My version: 7.1.3570.39 (Stable channel) (64-bit)
Might be time to finally move to Firefox though, if Vivaldi doesn't keep Manifest V2 support.