1-4a rename
Excellent freeware file renamer for large file numbers.
One For All

Can 1-4a Rename save undo files to undo mistakenly renamed files AFTER 1-4a Rename has been closed? Not yet, but this seems to be a good idea, just like .bak files where you still have the old version of the file. I guess, I will implement something like one or more .bat files with all the renamings needed to undo the last renaming(s).