How to change an entire path

The easiest way of doing this is simply to rename the backup disk (in the Finder while Lightroom is not running). Change its name to the original disk, and Lightroom Classic won't know the difference and happily use the new disk as if it were the old one. Do rename (or unmount) the old disk before you start Lightroom Classic. You don't want to confuse it with two disks having the same name. 

No remaking catalogs, no importing at the new location. Just re-addressing what you already have, which can usually be done in a few seconds as a single action.

Typically the file system holds a whole 'tree' of image folders, and this has been bodily copied or moved to a destination. In the Catalog, is a corresponding 'tree' of image paths referring to these folders and to the files inside those, still referring to the source location for your copy or move.

Every 'tree' of folders has by definition one common parent. This parent may or may not be currently showing inside the Catalog. For example you might see only "year" folders as the top level items of that drive volume, in the Folders panel. In the file system, there will be some higher folder level above.  Right-clicking on one of the 'year' folders you can choose "show parent folder" and reveal this higher level inside the Catalog. That common parent folder can now act as a single point of control for all these 'year' folders - or whatever else you have - and for everything inside those.

Right-click on that point-of-control folder and choose "update folder location". Browse as prompted to the corresponding folder on the other drive volume.

Everything below that - the whole 'tree' - is immediately re-addressed accordingly. Provided the identical folder arrangement has been transferred, naturally.

Or if you had (say) moved only one year's worth of photos - one branch of the 'tree' - to another drive volume, you would only re-address that one 'branch'.

All of this is equally possible regardless whether this has been a move, or a copy. 'Show parent folder' still works inside the Catalog, even if that folder is no longer really there in the file system, because what the Folders panel shows depends only on the Catalog's retained records, not on the live file system. One detail: "update folder location' changes to 'find missing folder' in this case - but you would still re-browse in the same way; then everything in the Catalog reconnects using the appropriate new, working paths and you are back in business.