Shame, I have a work around for it.
Create a new project, and transfer all of the data from the old demo file into the new project file. Everything will be there.
One downside, though, is that RPG Maker VX Ace will become stupid and won't let you view the graphics files. This has happened to me with some Ace games I downloaded. They let me check out the music used, but not the graphics. That may be your only problem, but you can easily get graphics back.