That will work for your current active version. You already can’t remove 2019 from one machine and activate it on a new machine, if you had previously activated 2020. But you could remove 2020 and activate that on a new machine.
But, you raise an interesting question about whether the license server will remain online. There are big institutions who are only permitted to use earlier versions of SketchUp. The license server has got to stick around for those guys, many of them have lots of weaponry they could point at us. I think you’ll be ok.