Because we all know Harry potter and the films. Yes everybody would understand. But it is not a good sentence, because what you have actually said is "Harry potter was a book" --> then he metamorphosised into a book. (changed/become a film). I would agree with Sarah's sentences.

I agree with Sarah above, but I'd like to add that since Harry Potter is so well known "Harry Potter was a book before becoming a movie" sounds natural to me. If it were some other name I'd never heard of then it wouldn't work as well.

I might say this: "The Harry Potter books came out before the Harry Potter movies." Or this: "There were Harry Potter books before there were Harry Potter movies."