Not sure why you'd find this so interesting, but here's the news:

I created a new Excel file, and opened one of the previous files.
Right-clicking on a worksheet tab allowed a "Move or Copy", and pops
up a small dialog box to indicate the name of the receiving file,
where in the list of existing tabs you want this sheet placed, and
whether you want a Move (which deletes from the curent file) or a Copy
(doesn't deleted).

I first attempted to move each tab, one at a time. After they were
all moved, I tried a Save As, which failed (same as before). I
figured one of the tabs has something corrupted in it. So I started
again, moving each tab, and doing a Save As after each.
Interestingly, I got through the whole list (there's about 20 tabs),
without a failure. This took a bit longer because many tabs surpassed
a 255 character limit, and were truncated. (Don't know why this
didn't happen in the first go-around.) So I had to edit each tab with
truncated data, delete the contents, select and copy the originating
tab's data, and paste into the new tab. Getting to that point took
about 2 hours. That's as far as I got yesterday.

Later today, I'll see if I can copy/paste the code for each form and
module to get a working app again -- and try another Save As.