The MessageClass property of each item determines what form Outlook will use
to try to display that item.

The form is just a template. It's never actually being "used" directly in
the way that you're probably thinking.

Each of the tools listed modifies individual Outlook items to change the
value of their MessageClass property and then save the item. You will want
to perform the operation when no one is using those items. If you want to
see how it works, use one of the script tools and read the code.