Demos are great to crstallise thoughts and really build the platform for your application. Don't underestimate the value of the experience gained by putting put a demo. Afterall, you want to build something people use - you can't do that with an idea.

We've got a number of applications around the same core bunch of ideas. prototyping one application and getting people using it couldn't be a better validation of one's ability to execute when it comes to Y combinator. Lying back on an idea and waiting doesn't show anyone you want to make it happen.

If your idea changes - well it just means you've proved you can execute before. Get a demo out even if its very rough (anything is). Just start!