At the moment a user is not able to change between different database backends in the Visual Designer e.g. you can only work with one backend in one persistent model. However that is about to change for the Q1 2011 release coming out in March this year. We will put functionality which would allow you to configure your model for different backend during design time and then just switch the current backend during runtime.
We will make sure to update you as soon as the functionality is available and documented.
the Telerik team