I am fully remote and have worked for this company for over 5 years as a W-2 employee. I got permission to move to Utah in January 2025 to continue as a fully remote employee. When it comes time to file my taxes for 2025 (in 2026), will I file California state income tax or Utah or both? There are still CA withholdings on my income. Do I file both and file a credit for income tax paid in another state form in Utah? My HR department won't remove the CA withholdings. The only other option they have given me is to become an independent contractor and that is confusing and worries me.
Yes, you will file for 2025 as a Utah resident and report all your income. You will be able to report any tax you paid to California in your UT state interview, as tax paid to another state, as you mention.
You will also need to file a California Non-Resident return to report the income you earned from CA sources.
Here's more info on How to File a Non-Resident State Return.