I'd be surprised if there isn't some way to do it in Excel with a little fiddling, but I don't know enough about Excel.You could select the relevant cells and copy them to a text editor and do the change there.
If you don't have one, are desperate, and only want it for this one emergency, you could get [invalid URL removed]Vim. ( ftp://ftp.vim.org/pub/vim/pc/gvim70. exe )
To anyone not familiar with it, it's a really weird editor; but it's not a huge download and it can do the case toggling you're after.
After you've installed Vim, do the following:
- Start Excel and open the spreadsheet.
- Select the cells with the text you want to modify, and copy them to the clipboard (Ctrl-C)
- Start Vim. (gvim. exe)
- Press these keys as written (including the double-quotes): "+Pggg~Ggg"+yG
- Switch back to Excel and paste the modified text back into the spreadsheet (Ctrl-V), overwriting the still selected cells.
Yes, I am half way kidding. I don't expect you to do all that (someone please tell her how to do it in Excel) , but if it's a lot of text and you really are desperate, and if no-one else drops by here to tell you how to do it in Excel itself, it will work.
For the curious geeks, the key sequence I wrote above does the following:
"+P pastes the text in the clipboard into Vim's buffer.
gg moves the cursor to the top of the buffer. (It was probably already there, but I'm typing these instructions from memory, so I'm just making sure.)
g~ toggles the case between the cursor's current location and where the cursor will be after the next motion command.
G is the motion command to move the cursor to the end of the buffer. The text will now have toggled case. (Previously lower case letters will now upper case, and upper case will be lower case.)
gg moves the cursor back to the top. (Again, I can't remember if this is necessary.)
"+y copies the text between the cursor's current location and where the cursor will be after the next motion command into the clipboard.
G is (once again) the motion command to move the cursor to the end of the buffer.
Good grief, I'm a geek.
BTW, typing gu instead of g~ would have turned all the text into lower case, and gU would have turned it into upper case.

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