I've been using Amy Dullard's great batch file for switching the tools back and forth between Silverlight 2 and Silverlight 3. It works great except that it doesn't change the runtime. And while the Silverlight 3 runtime *should* run any Silverlight 2 code fine, I just don't trust it. So I had to make it better.
I've made a small change in Amy's batch file to uninstall and install the right runtime, but there is a hitch. You need to download the runtimes yourself (as they are downloaded during installation and since Amy's batch is opening up the Setup package, they aren't there).
To use the batch file you'll need to download the file (here) and unzip it into an empty directory. Into that directory copy the following files:
You can download them by clicking on the links.
If you haven't done so already, run the Silverlight 2 Tools Package and the Silverlight 3 Tools Package. Finally you can run the Setup_SLTools_Util.bat file that is in the zip file. This will create new batch files for moving two and from SL2/SL3 for Visual Studio and Visual Web Developer Express. Please see Amy Dullard's post about it for more information about how the batch files are used: