PDE provides an Update Classpath wizard that quickly updates the classpaths of multiple projects. The update will change the JRE and compiler settings to best match the first execution environment specified in each project's Manifest. The wizard is available under the Plug-in Tools menu after right clicking on a Plug-in Project.
When updating classpaths, you may select any plug-in or fragment project in your workspace. All selected projects in your workspace will be selected in the Update Classpath wizard by default.