Backup Metadata from old empty folders

There are two places where photo metadata lives: in the Catalog, and alongside the Raw source files (or inside the source file for other kinds of imported image). There is no "sync" metadata command, by the way, only Write metadata from Catalog to file, and Read metadata from file to Catalog. And when you Write metadata, only some of the information held by the Catalog gets included. 

If you have got a Catalog where the images are (still) showing as imported, then there is no need to write out metadata: you will need to make sure your Catalog is backed up and is then back in place again, after your system change. This puts everything back as before, including all of the info which could not have been written out anyway.

If you had not got a Catalog where the images were (still) showing as imported, then you would not have any Catalog metadata which could be written to the images files. But that doesn't sound like the situation.

As well as making sure you have a backup of your Catalog and of all your image files, it is also worth making sure you have backups of all your presets, watermarks, any downloaded or generated camera and lens profiles etc. These are stored in two central folders under an "Adobe" location in the computer user's files area: one called CameraRaw and the other called Lightroom. To find this location you can e.g open the Presets location in your file browser, from the menu at top of Presets panel (or else from inside LR Preferences).  

Are these photos still cataloged in the Lightroom Catalog?