Sorry.., the term I'm sure is *Break Link*!

Thank you. Because I'm German I had to translate this, and googling for a
propper term I found the solution:

If a table in the active Excel file contains an external link the menu item
"Edit - Links" is enabled and this function will allow to change references
to an Excel table in an external file to an Excel table with the same name
in another external or in the active Excel file.

So I could copy the sheets of the damaged .xls-file into a newly created
empty file very easy by Strg + dragging them into the new table, save it to
disk, and change the links in one step as described above, and the annoying
"file was created using a later version." has gone.

It's so simple if you know it...

Thanks,
wuez