From the XNA Team Blog's post: "... We’ve gone over many of the features outlined in the announcement we made at GDC 2007. One thing we didn’t really cover previously is compatibility. The entire team made a conscious effort to maintain compatibility with this release.
Your game should just recompile in 1.0 Refresh with no changes required. Any game that is compiled should run fine if you have 1.0 Refresh installed.As mentioned, you can continue to use 1.0 on your Windows computer with 1.0 Refresh on the Xbox 360 console.
Games that are built on 1.0 Refresh may run on 1.0; however, if you are using new functions or features in the XNA Framework (such as font functionality), it will not work properly.You can download the XNA Framework 1.0 Refresh for including in your installer as you start to upgrade your games ...".Donwloading it right now!