Metadata and Collections

Quote, “and this 20+ year old Catalog has many legacy issues.”

Lr version1 was released in Feb 2007 so not quite 20 just 15.

I agree with the comments by Canrad and Richard.
A backup of the Catalog, and the image files in the folder structure, is all you need.

You are bound to get Catalog based methods suggested first here, simply because this way your complete information is maintained. Most importantly (library organisational aspects aside) any Virtual Copies present, and all prior editing History, would get lost in the course of the reorganisation proposed.

That said, perhaps there are no Virtual Copies at all and perhaps you are not interested in keeping previous editing History, or any of the organisational stuff except Collection membership which has been mentioned. One approach for expressing Collections into externally written metadata could be to write those to the images concerned, as special keywords (perhaps with a standard prefix to separate them from other classes of keywording). Perhaps you open each Collection in turn, and highlight all its images, and give those a special keyword naming this collection. At the end of all this, each image will have accumulated a keyword for each Collection it belongs to. When then imported into a fresh Catalog either a Smart Collection can group those images based only on having this same keyword, or else images with each such keyword can be searched for and then added to a correspondingly named standard Collection.

All this seems to me no less laborious to complete, than sorting out the images as a Catalog library. Perhaps breaking down year by year to keep things manageable: select all images from 2002 and Export those selectively As Catalog: physically reorganise the folder locations of the 2002 images, then do the same process for all images from 2003, and so on. At the end merge all those intermediates into one new overall Catalog. Or some variation on that: food for thought, at least, I hope. 

Collection membership is one of those things that is stored only in the catalog, because it really is a catalog level thing. For example, although it should be technically possible to have something in the metadata that says what catalog a photo belongs to, a catalog also stores collection sort order, collection set info, and other things that can’t be stored in one photo because they’re about relationships across multiple photos and collections.

I don’t like losing catalog-level information either. So if I was doing this, I would start a new catalog and then import the old one. (Basically the opposite of exporting from the old catalog, but with the same result.) And then, if some folders are so large that Lightroom Classic seems to take too much time moving them around, I would move one folder at a time on the desktop, and then after each folder move, I would switch back to Lightroom Classic, and use the Update Folder Location command to show Lightroom Classic where that folder went. That way, the move is done by the OS, and Lightroom Classic just needs to be shown the new location so it can update all of its links to the images.