It was my understanding that while the projectors were upgraded to HD, the projections themselves were not, and that when Fantasmic comes back up the film itself will be in HD and shown on the HD projectors. Since the projectors were updated it's been like when you have an HD TV and watch a non-HD channel on it; some things are a little better looking, but on the whole it's not too different. Then you put on an HD channel and you can see the difference. Sometimes. In short you should see a difference if you're that familiar with Fantasmic's previous projection quality.
But yeah, that was how I understood it would be going, but that's just my recollection and it may be wrong.