Why can't I use my older Xcode version after an upgrade?

I had been using Xco开发者_StackOverflow中文版de 3.1.3 on Leopard, but I just upgraded to Snow Leopard (10.6.4) and upgraded my Xcode to 3.2 with the iOS 4.0 SDK. Now I'm not able to go back to my older Xcode version to build projects for an older OS version.

How can I build applications that target my original model iPhone, which doesn't support iOS 4.0?

There was options to install xCode version at the time of installing xCode Please also select xCode 3.1.3 and 3.0 at the time of installing.