changing default applications


I want to change the default application for opening RTF files from Word to AppleWorks. So, I select an RTF file in the Finder, command-i, and go to Application. But the "Change Application" button is ghosted out - it won't let me switch.

Is there any other way to change it, or am I stuck with Word as the default application here?
Well, I *should* be. Although I saved the files in question in RTF form on a PC, not on my computer - maybe that's screwing it up?