Major revision and re-submit as a Technical Note
To make it clear to the editor, formally withdraw it from consideration. Then it is yours to do with as you please. They have no hold over you or the paper until you formally sign copyright away. What you have sent is likely enough, actually, but a formal notice of withdrawal leaves no ambiguity. 

In most places I think that giving notice is enough and their reply isn't needed. But you need to keep records. Formal mail (sneaker net) with a receipt for delivery is sometimes used for such things. And keep the receipt.