Ok ... so the SS# was issued prior to your filing the return ? And you used the ITIN by mistake ?
If so not sure if the IRS is correct in this matter but if you do amend the 2019 return just to get the stimulus check then I think you will be sadly disappointed as amended returns are taking 6 months to process and until it does the stimulus will not be issued. But I you do want to mail in a 1040X then the amounts in columns A&C will be identical and column B will be blank. On page 2 you need to explain why you are sending the form.
I highly recommend you only mail in the 8822 at this time and wait for the credit on the 2020 return that will be filed in Jan 2021.
Also if you have not yet done so make sure to alert the IRS about the ITIN to SS conversion for record consolidation.
