Finally moving from a perpetual LR copy to LR Classic

There is nothing you need to do with the Catalog or with the photos. When you install and run the new software version, signing that in to your Adobe Account, it may open to a fresh empty Catalog. But you should then Open your prior Catalog instead. This will prompt for the name of a new, converted copy Catalog (by default, in the same place but you can change that location if you want) which will then open.

Inside you will see everything the same. The same images organised in the same Collections and Smart Collections, with the same image attributes and edits, stacking and such. Local adjustments will be replicated using the new Masking system, to give the identical visual result. And this is all referring to (sharing the use of) the same source files on disk as the old Catalog continues to refer to, with no duplication of those. That old version Catalog can simply be taken out of use, and deleted sooner or later.

Couple of things to be aware of:

in order to show everything the same, individual images will continue to adjust under the same Process Version as previously. You may want to bring particular images up to the latest process version if you want to use the latest edit methods on those (click lightning-bolt icon just under Histogram in Develop). 

Also, you may notice that new Raw imports begin a little more contrasty and vivid than they did under older software versions. That is the new "Adobe Color" profile which becomes the import default - but can be changed. Profiles and processing defaults do work a little differently, also develop presets will be converted to a new format and location.

https://www.lightroomqueen.com/how-move-lightroom-to-new-computer/

You can simply open your catalog with the new Lightroom Classic version. It will make a copy at the same location and then upgrade the catalog. If you move off from the external drive, it's important that all your pictures stay at the same place related to the catalog and the drive has the same letter to start off. If everything works as expected, you will not see any question and/or exclamation mark on the folders/pictures.