One interesting take on the story changes, which I got from xiphias's post about the movie: Except for some relatively minor things, all of the plot changes are ones where nobody was there but Beowulf -- in other words, the version in the movie could well have given rise to oral-tradition tales that became the real-world myth, if Beowulf told a few self-aggrandizing tales about what happened.

I haven't seen the movie, so I don't know how accurate that is, but it seems like a quite nifty idea nonetheless. Especially if they're portraying him as a "deceitful, boastful publicity hound".