New Receiver
 

I just bought a Pioneer VSX-D514 receiver. It's a 5.1 system. I don't have a surround setup though, I just have two 15" speakers connected.
If I were to upgrade to surround sound, would that make my 2 speakers obsolete? The new receiver doesn't have a speaker switch, so I'm assuming there's no way to go from stereo to 5.1.

You could buy three new speakers and a sub. Use your current two speakers as the rears.

The front three speakers should be "matched" for best results. IE: The same line from the same manufacturer.

(You can ignore what I just said if your front's are expensive speakers. Then you should buy a center from the same manufacturer and get two small speakers for the rears. The subwoofer can come from anywhere.)