I'm glad that you found the earlier article How to add Artwork to your iTunes Movies useful. Turns out that you were just about exactly where you needed to be to also recategorize your videos from TV Show to Movie when you were in the "Get Info" window.

You'll recall that when you were adding Artwork, there were a bunch of different tabs along the top of that window, namely: Summary, Info, Video, Sorting, Options, Lyrics and Artwork. The categorization you seek is in the "Video" tab.

To get there you need to Ctrl-Click on the movie in iTunes:

Choose "Get Info" as you did last time, but this time, click on the "Video" tab, rather than the "Artwork" tab. Right on top is the pop-up menu you want, labeled "Video Kind":

Instead of "TV Show", simply choose "Movie" and you're done. Click "OK".

You can simply do this one at a time for all the videos you need to move over. And, as you can see, this also is how you categorize music videos too, if you want to be really neat and tidy.