mkay...

...seems all was not as simple as I first thought.

Came up with over 1,100 "Missing Photos" in each of my two now-synchronized catalogs.

erk...

But, Lightroom to the rescue.

Select a specific missing photo (part of the beauty of Lightroom is that, even though it knows when a photo's gone missing it can still show you what it knew about it before it vanished) in Grid mode of the Library module, and click on the "Missing Photo" icon in the upper-right corner.

Lightroom attempts to give you a best-guess as to where it was, to navigate from.

*If* you know the photo is really still around, navigate to the correct folder and select the specific file.

Leave "Find nearby photos?" selected.

Click OK.

Lightroom recovers the missing photos!

In the Library module, under "Catalog" select "All photos" and select Library > Find missing photos.

Again, select one, click "Missing" icon, find, find nearby.

Find the photos, Spot! Good dog.

Repeat as needed, or wade through all the missing photos all at once.

Coolness...

And what had happened to the "Missing" photos in the first place?

I had renamed a few directories on my file server, which Lightroom had no way of knowing about.

Takeaway: only rename source directories on the file server from within Lightroom, under the "Folders" panel in the Library module.

duh...