I am not aware that the IRS has prescribed a format for requesting they combine your tax records under a newly-issued SSN.
It will suffice to state:
To the IRS:
My ITIN is XXX-XX-XXXX.
I was issued a SSN on [Date - XX/XX/XXXX].
I will immediately cease using my ITIN for tax reporting purposes.
Please combine any tax records shown under my old ITIN with those showing
under my newly-issued SSN.
