Hate to tell you but you should NOT expect any stimulus checks this year ... amending the return for a change from ITIN to SS# after you file will not work at all. The stimulus checks are really a 2020 credit that you will get on the 2020 return when file it next year if you qualify ... so you will not lose out on the payment you will simply need to wait a bit longer.
You need to notify the IRS, so it can combine all of your tax records under one identification number. This is from IRS website:
What do I do when I am assigned a social security number (SSN)?
Once you receive a SSN, you must use that number for tax purposes and discontinue using your ITIN. It is improper to use both the ITIN and the SSN assigned to the same person to file tax returns. It is your responsibility to notify the IRS so we can combine all of your tax records under one identification number. If you do not notify the IRS when you are assigned a SSN, you may not receive credit for all wages paid and taxes withheld which could reduce the amount of any refund due. You can visit a local IRS office or write a letter explaining that you have now been assigned a SSN and want your tax records combined. Include your complete name, mailing address, and ITIN along with a copy of your social security card and a copy of the CP 565, Notice of ITIN Assignment, if available. The IRS will void the ITIN and associate all prior tax information filed under the ITIN with the SSN. Send your letter to:
Internal Revenue Service
Austin, TX 73301 - 0057
