I am looking at buying the low end model mac mini, and I wondered the same thing. I know it is possible though, as I watched ifixit's teardown of the new mini on youtube as well as visiting their website. The teardown shows that with the right tools, you can get at the Hard Drive, and replace it/switch it easily.
Tools you need
-Pry tool/flat head screwdriver
-Torx T6 Security screwdriver