In case you haven't tried Dave's utility yet and so you can learn a little bit about how Tag Studio works, here's how you do what you asked:

First, set your working directory for where you have your MP3s.

1. Under "Mass set tags/flags", click "Use filtering".
2. Click "Setup".
3. Check the box for "Filter tag contents".
4. Check the box for "Artist"
5. Check the box for either "Equal to" or "Containing".
6. In the text box to the right, enter "Joseph Hayden".
7. Click "Ok".
8. Check the box for "Artist".
9. In the text box to the right, enter "Joseph Haydn".
10. *IF* you have subdirectories containing more Joseph Haydn MP3s, check the box labeled "Incl. subdirs".
11. Click "Execute!".

That's it. If you mess it up, just click "Undo" and everything will revert back. The filtering capability of Magnus' Tag Studio is unreal. (BTW... if you haven't purchased it, I'd urge you to do so. It's definitely worth it, if you use it on a regular basis.) Copy some MP3s to a seperate directory and do some experimenting with the program. That's the best way to learn. Let me know if you need any more help using his program.