It seems there is a nasty bug (or feature) in the Lightroom Classic CC installer. If you install the upgrade, you would expect that the previous version (Lightroom CC2015) is kept, so you can perhaps first play around with a newly created catalog, or go back to Lr CC2015 and your old catalog copy in case of problems. The installer removes Lightroom CC2015 however and it also disappears from the list in your CC app, so I wouldn't know how to reinstall it (except from a local backup...).
There are also reports from Lightroom 6 (perpetual license) users, who mistakenly thought that this was an update for their version already, who lost their Lightroom 6 application and got a trial Lightroom Classic CC version instead...
There are also reports from Lightroom 6 (perpetual license) users, who mistakenly thought that this was an update for their version already, who lost their Lightroom 6 application and got a trial Lightroom Classic CC version instead...