In lieu of a script, you can select multiple albums and change their genres all at once.
If you go this route, try this to keep track of what you've done:
1. Make a smart playlist of everything. Give it a name you're good with.
2. Choose the first genre you want to fix; for this example "Pop"
3. Add this rule to your playlist: --Genre-- --is not-- --Pop--
4. Everything that is already Pop should disappear (anything that contains Pop but is not specifically "Pop" will still be there, e.g. "Pop Punk"
5. Highlight everything remaining that you want to change to Pop. Right click and choose "Get Info". Change the genre to Pop.
6. After processing, everything that was changed should disappear from the playlist.
7. Add the next genre as in Step 3 and repeat; don't delete from the playlist rules the genres you've already fixed.
8. When the playlist is empty, yer done.