After using iTunes for some years, my library has become too fat even for my ever-growing disk sizes. I decided to move my videos to another drive and found there is an incredibly easy way to achieve this: use the Finder to go to your iTunes library folder and move the “Movies” and “TV Shows” folders to the new drive – done.
iTunes automatically (!) found the new location without any manual re-indexing or re-importing or fiddling with the library preferences. (I have not tested network drives.)
There is a catch, though: downloading new videos – e.g. TV show subscriptions – will recreate the folders in your iTunes library. If you find that confusing, I suggest you only move complete seasons/subfolders.