The stimulus "RECONCILIATION" will be part of the 2020 tax return ... nothing you need to do extra.
If you used an ITIN on the 2019 return and now have a SS# you must tell the IRS so the tax return records can be consolidated ... if you have not yet done so do it ASAP so the 2020 return is not delayed.
https://www.irs.gov/individuals/additional-itin-information
