an interesting issue. certainly, you can change the CLCO to the correct amount on the 2019 return without filing an amended 2018 return unless the amount was material (don't ask for what material means. the IRS doesn't have one. it's more CPA terminology that can roughly translate as substantial but as to a $ or cent number it will vary from CPA to CPA). since it'll be lower the IRS might not inquire. if it does then you can submit with the notice an amended 2018 return. it's rare to have a transaction like this that doesn't affect something else. the exception being a year-end stock dividend where you were entitled to whole and fractional shares but the company would not issue frac shares so they would be sold. or it could be a transaction that occurred in 2019 that was really reportable in 2018. I would make sure the transaction isn't also reported on the 2019 1099-B.
