@Gweman
Supposedly, either 19 or 20 February.
You can't stop your original tax file, so wait until you get the original refund (or have paid the original tax due) before you start preparing an amended tax return.
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BUT....if your original tax filing is not yet "Accepted" or "Rejected" yet....especially don't do anything right now. IF it's "Rejected" then you just EDIT your tax file after the rejection and re-submit.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*