"everyone wants to turn an estimate into invoice."

Because Estimates = quote or proposal, or Scope of work for price, as a step Prior to the actual charges.

You don't go from Invoice (actual Charges) to Estimate. You can create an estimate and use it for good comparison reporting, but is is a bit late to try to Progress Invoice; you already invoiced the customer.

"open your invoice and change the template to the Estimate."

You cannot do this; the templates are matched to their Transaction Type.

"Print or e-mail it.  mark it Pending."

Pending invoices are replaced by Estimates about 20 years ago. And if this person issued their invoice, that customer has been charged and that is a sale. Making it Pending after the fact changes all the financial reporting.