I HAVE THE SOLUTION!

I've been researching a way to convert an invoice to an estimate for months (and even had several calls with Quickbooks Support and they said it was not possible with QBO).

Well, I figured out a way to do this tonight and wanted to share with the community.

Go to www.saasant.com and Export the Invoice(s) you'd like to have in QBO as Estimate(s). 

Then go to Import > Estimates (and choose the Invoice(s) you just exported). 

Now a window will come up with fields to match, ensure you revise as needed - I only had 3 fields to choose a drop-down for.

Then complete the Import.

Now you'll see those 'Invoice(s)' in QBO in your Estimates list!

This saved me dozens of hours of work, hope it helps you as well.