I find this interesting because, apart from being sufficient a change to make it actually an adaption of the book (as it stands it has more in common with 28 Days Later than the book than it supposedly adapts), it clearly was the ending intended when certain scenes were filmed, and contains the pay-off for various curious aspects we had noted in post-movie discussion. we had noted that the lead zombie seemed to have some connection to the captured female zombie and clearly wanted to get her back, and also that the butterfly bit was a bit random. both things make more sense here.
It isn't quite so saccharine, but is happy enough that it ought to have been possible to release the movie like that without worrying the studio too much. Oughtn't it?