Do I keep the same tax status all year, or can I be nonresident and resident in the same year?

You can be both in one year, which the IRS calls a dual-status alien. This commonly happens in your arrival year on H-1B or after getting a green card: you're a nonresident up to your residency starting date and a resident afterward. In the nonresident part you're generally taxed only on U.S.-source income; in the resident part you're taxed on worldwide income. Dual-status filers face special rules, can't take the standard deduction, and can't file jointly unless they make a special election. It's one of the trickiest immigrant tax situations, so many people in it use a preparer. See IRS Publication 519's dual-status chapter at irs.gov, and consider the tax-health assessment at wealthserene.com/assessments/tax-health.