Change item template

  Item mainEventItem = Sitecore.Context.Database.GetItem("{C72639F9-8AC7-43C1-9154-DA72C396FE7E}");
      TemplateItem template = Context.Database.Templates["{1C041906-1194-4498-81B2-E7399ECC4908}"];
      Item newTemplateItem = null;

foreach (Item descendant in mainEventItem.Axes.GetDescendants())

        {

          if (descendant.Template == null)

          {

            newSheet.Cell(newRowCount, 0).Value = descendant.Paths.FullPath;

            newSheet.Cell(newRowCount, 1).Value = "missing template";

            newRowCount++;

          }

          else

          {

            switch (descendant.Template.InnerItem.Paths.FullPath)

            {

              case "/sitecore/templates/Components/Events/Event":

                newTemplateItem = Context.Database.GetItem("/sitecore/templates/Project/Corporate/Content Types/Events/Event");

                template = Context.Database.Templates[newTemplateItem.ID];

                using (new SecurityDisabler())

                {

                  descendant.ChangeTemplate(template);

                }

                break;;


NOTE: if item is unpublishable and I am in live mode (just master) then it skips those items. Use power shell to delete those.

cd 'master:/sitecore/content/Corporate/Home/Learn/Resources Library'

Get-ChildItem -Recurse . | ForEach-Object { 

    $isPublishable = $_.Publishing.IsPublishable([System.DateTime]::Now, $true)

    if (!$isPublishable) {

        #Get-Item $_.ID | Remove-Item -Recurse

        Write-Host -f Red $_.Paths.ContentPath $_.ID is unpublishable

    }

}