try this script
dougscripts.com has a lot of other useful scripts which you can use inside of iTunes.
http://dougscripts.com/itunes/scripts/scripts01.php?page=1#setvideokindofselected
Download the script and place it in ~/Library/iTunes/Scripts
All of these will show up in the Scripts menu in iTunes.
Once you have it installed, import your video into iTunes. Select the ones you want to change. In the menu bar, click on the Scripts menu (it's a little icon by Help that only shows up when you have scripts installed) then choose the "Set Video Kind of Selected". Double-click on the available fields to change them.
This is the quickest way to do it. I wish they'd just add this into iTunes somehow.
You could also use an app called Lostify.
http://lostify.com/download/
It actually changes the metadata in the file by making a copy with the new tags or rewriting over the original. You can also add a script for this into iTunes. In iTunes, highlight the file(s) - Scripts menu - Lostify - Lostify will open change the file and update iTunes with the new info.
Let me know if you have any questions.