Technically, you are considered as a dual-status alien. ( January 1st - 15th as a nonresident and rest of the year as a resident)
As you file a Form 1040 with TurboTax, you would have reported worldwide income including the period of time before you entered the US.
-If you have received any worldwide income or if you only receive US source income for the first 15 days ( January 1st- 15th) and did not report them, I would suggest you submit an amendment. (Make sure you only report US source income on the Form 1040-NR)
-If you did not receive any income for that period of time, I would not do anything until you receive a correspondence from IRS requesting any additional information.
To amend from Form 1040 to 1040NR, here are the steps:
As TurboTax does not support any nonresident tax forms, you are recommended to use our affiliated partner, Sprintax to complete your Form 1040-NR.
1. Download the Form 1040-X from IRS website ( see below). On the front page of Form 1040-X, enter your name, current address, and social security number (SSN) or IRS individual taxpayer identification number (ITIN)
2. Do not enter any other information on page 1.
3. Do not complete Parts I or II on page 2 of Form 1040X.
4. Enter in Part III the reason why you are filing Form 1040X.
5. Complete a new Form 1040-NR by using Sprintax, see above.
6. Across the top of the 1040-NR, write “Amended”
7. Attach the new Form 1040NR to the back of Form 1040X
For more information, see:
https://www.irs.gov/pub/irs-pdf/f1040x.pdf - form
https://www.irs.gov/pub/irs-pdf/i1040x.pdf - instructions
page 5 under Residents and Nonresidents
