After creating a project membership, you can always go back to editing it again. For single project memberships this is done by clicking on the ellipsis icon and choosing the "Edit project membership" option:
You can also mass-edit project memberships in Silverbucket. This can be done in any of the views that list multiple memberships for you at once: project's planning view and the main views for people and projects, for example.
Select your memberships and choose the "Mass edit project memberships" under the ellipsis icon located just top of the listed memberships. If you wish to select all, you can first click on the ellipsis icon to select all and then click it again to perform a mass-edit. Careful with this, as this will affect every single project membership listed below; this action can not be reversed.
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Note: if you inactivate a membership, its future resourcing plans will be deleted.