Amending Form 1040NR to Form1040

[Murtaza_Ismail_CPA]

Hello; 

The prior preparer filed Form 1040NR for tax year 2021 for my married client as Married Filing Separate. 

According to my research, my client met the rules of Substantial Presence Test / US Resident, should should have filed regular Form 1040.

Facts:
He entered USA on work visa on 06/29/2021 and remained in the country until end of the year (186 Days). I think he is considered a US Resident under Substantial Presence Test rule.

His wife entered USA with spousal work visa on 08/25/2021 and remained in the country till 12/31/2021 (129 Days). I think she is not considered a US Resident under Substantial Presence Test rule. However, US resident can elect to claim a non-resident spouse as a resident for tax purpose. (https://www.irs.gov/individuals/international-taxpayers/nonresident-spouse). 

Both of them were issued SSN in 2021.

Question:
To convert Form 1040 NR (Married Filing Separate) to Form 1040 (Married Filing Joint), do we file regular Form 1040X? Can anyone help with exact procedures on Amendment Process?

Thanking everyone in Advance. 

Sincerely, 

M. Ismail

[BobKamman]

Your research didn't get very far if you didn't make it as far as Page 12 of Pub 519.

You generally make this choice when you file your joint
return. However, you can also make the choice by filing
Form 1040-X, Amended U.S. Individual Income Tax Return.

Attach Form 1040 or 1040-SR and enter “Amended”
across the top of the corrected return. If you make the
choice with an amended return, you and your spouse
must also amend any returns that you may have filed after
the year for which you made the choice.