A useful tip has been published by Amy Dullard, about how to install Silverlight 2.0 and 3.0 on the same machine an continue working with both. You have to take note that the Silverlight 3.0 runtime will override the Silverlight 2.0 runtime but due to the big work in backward compatibility this would not be a big problem.
The tip require a batch script to switch from a version to another of the Visual Studio Tools. Included in the tip there is also a way to change the projects to work with both versions of the tools.
Link: Switching from Silverlight 3 Tools to Silverlight 2 Tools