My employer withheld social security and medicare taxes after my status changed from F1 to H1B? Can I claim those in refunds? If no, how do I show that I was eligible for exemptions during my F1 while filing my returns?
I agree. But while filing 1040NR or 1040NR-EZ, how do I show that I was on H1B status only from October 16 to Dec 31, 2018? How do I calculate how much I owe/ how much I'm owed based on my change of status in October?
I wish I could assist you here. However, we do not cover information regarding nonresidents or form 1040NR related. I would suggest you call or contact <a rel="nofollow" target="_blank" href="https://www.sprintax.com/">https://www.sprintax.com/</a> for further assistance.
Yes, you do need to pay the FICA taxes.
Per IRS, there are only certain kinds of H-visa that are exempt from the FICA taxes. Once you change to an H-1B visa, you are subject to FICA taxes. See FICA taxes for visaholders.