In Standard View>Video>Files view, add a column for "media sub type" (you can right click on the column header and add any field from the list shown). Then you can change the media sub tag from "TV Show" to "Movie" or vise-versa by selecting the file and hitting F2 key to edit that row.
When you setup the folders to be "Watched" at Tools>Options>Library & Folders>configure auto import (adding each folder where you import your media from), you can have MC automatically apply the media sub type that you want.
For example: All of your Movies might be located at
\Movies\movieName and your TV Shows might be at D:\TV-Shows\seriesName
Then you would set up the auto-import to watch folder "D:\Movies" and "D:\TV-Shows".
Then in the configure auto-import setup (for each folder) you can use the "Apply these tags (optional)" to have MC
add the media sub tag (i.e. "Movie" or "TV Show", etc.) to everything that is imported from that folder. I have separate folders for Music, Images, Movies, TV-Shows, Music Videos.
If you don't tell MC what type of media sub type to use (as described above) I believe MC will assign "TV Show" to any video under 60 minutes and "Movie" to everything longer than 60 minutes.