You’ve already filed and accounted for the sale as short term. Now you've received a new 1099-B with the sale category changed from short term to “unknown.”
If the total proceeds amount has remained unchanged – merely shifting the amount from one category to another -- and your holding period was indeed a year or less (short term), then there’s no need to amend because you reported a short term sale and it was a short term sale.
If your total proceeds have decreased – not just the original short-term section decreasing, but the total of all sections (including the “unknown”) – and the sale is indeed short term as you reported on your original return, then you don’t need to amend. You can simply make the cap loss carryover adjustment in TurboTax next year when you file your 2017 return (Federal Taxes > Wages & Income > Investment Income > Capital Loss Carryover).
An amendment would be necessary if your total proceeds actually increased, but it doesn’t sound like that’s the case here.
