Nothing, just like all adaptations.
However, changes that would are necessary is changing, re-arranging, and taking out things/aspects of whatever they are adapting. For example, say the first movie was written so Frieza is the main villain. We could have Raditz being defeated, but Goku not dying. We could have Goku and Piccolo already being allies. We could leave out Vegeta and Nappa coming to Earth, and just have Vegeta along in Frieza's ranks waiting to betray later on. The movie doesn't even have to take place on Namek, etc.
In short, the point that I am trying to get to is nobody wants the source material changed, but fans must accept that things must be changed in order to suit the film's script. My answer to the question is whatever is fit for the film. Of course, I still oppose Dragon Ball being adapted because I feel it is simply too hard to adapt properly, even with necessary changes. Sadly, we already have ninja Nameks, Goku in high school, etc...