correct, you can re-download a flick that you had on youtube already. you can also use online tools like youtubedownloader or keepvid to grab someone-else's video from youtube back to your computer and save as mp4. In most cases the highest available version will be offered. Keepvid gives choices, in case you don't care for the 1080p version for example, or if you only want the audio track as an mp3

 

If you prefer to just do the conversion yourself you can use the free WinFF (powered by ffmpeg).