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$usrold =$web.SiteUsers.GetByLoginName($O365GroupOld)
          $ctx.Load($usrold)
          $ctx.ExecuteQuery()
          $web.SiteUsers.Remove($usrold)
          $ctx.ExecuteQuery()
          $usernew=$web.EnsureUser($O365GroupNew)
          $ctx.Load($usernew)
          $ctx.ExecuteQuery()
          $roleDefinition = $ctx.Site.RootWeb.RoleDefinitions.GetByType([Microsoft.SharePoint.Client.RoleType]::Contributor)
          $roleBindings = New-Object Microsoft.SharePoint.Client.RoleDefinitionBindingCollection($ctx)
          $roleBindings.Add($roleDefinition)
          $ctx.Load($web.RoleAssignments.Add($usernew, $roleBindings))
          $ctx.ExecuteQuery()