I think we simply need to understand the Impact, not the tasks available.

Turning an existing Invoice into Anything is impossible; there is no Next Stage.

You can Mark It Pending, which changes your financial reporting. You impacted sales and sales taxes, if you have them. We don't :)

It isn't turned into an estimate, because it is the Sale. Estimates come before the sale, or are created to offer comparison reporting.

There is no Going Backwards, from the sale, unless you want to Void or delete or undo the sale from the Financial reporting perspective, which only the user can determine is the proper thing to do.

You can create an estimate and ReDo the invoice, to get the same sales info, and avoid affecting financial reporting, as well. This is more of a Recreation using a different method.

It helps when people give us better details.