Man, if that was the case it would have been doomsday for film makers
That way even dial up users would download 3gp movies and convert them into high quality HD movies.
What actually happens in encoding (conversion) is that high quality movies (dvd,or any other) are stripped down by data to a level which is just enough for 3gp size (174 X 144) and rest of the data is lost. Hence 3 gp movies are just a fraction of high qaulity vids. The higher resolution the higher file size due to large data available pertaining to video.
So when you have a 3 gp video you cannot re-formulate a higher quality video since the data is lost
P.S. There may be softwares that convert 3gp to .vod (dvd) but thats just changing format to make it compatible with players. The quality cannot be regained
Converting dvd movies into avi format by using divx , xvid codecs is a different concept. That just try and remove redundant data without trying to loose much on video quality