How to toggle among different source control solutions?

It depends on the IDE you are using.

For the IDEs which read SCC info from the registry table (under HKEY_LOCAL_MACHINE>> Software>> SourceCodeControlProvider), such as Visual Studio 6.0/2003, you can use SCC Provider Manager. It can be launched from the SourceAnywhere Standalone Client program group. 

For the IDEs like Visual Studio 2005/2008, which keep individual binding information for each project and read SCC info from the binding information file, SCC Provider Manager will not work. You need to select the current SCC provider within the IDE.