[codethief]

I would counter that by suggesting that you create a second root partition (on which you then install Ubuntu 20.04) and remove the first partition (containing Ubuntu 18.04) when you're done. This is similar to how A/B upgrades work in recent Android versions. (Of course you'd have to have /home on a separate partition. Personally, I've been doing that for years, anyway, because I want to be able to reinstall quickly without overriding /home when a software upgrade happens to mess up my system.)