That's why I have two separate Ubuntu installations side by side. I currently have 16.04 and 18.04 but haven't booted 16.04 in a long time; it's still there if I ever need it, to check out some configuration I forgot to add to the newer Ubuntu. To make sure not to make a mess, I have two separate Home folders, with symlinks from one to the other for folders.
When Ubuntu 20.04 LTS comes out next year, I'll install it over the 16.04 partition and it will become my main system. I've been doing this since Ubuntu 10.04.