I'm currently maintaining an application built on Websphere 5. It's such a shame that once you select a default workspace, there would be no option inside the IDE to switch workspace(RAD7 and plain old Eclipse has this). Luckily, I had a friend who already experienced this kind of problem and he gave me this CLI command
wsappdev -setworkspace
Just be sure to run wsappdev from your websphere install directory and once WS starts, it will prompt you to select your workspace.