My domestic partner has health issues that prevent him from working full time, requiring me to pay to care for us. Could I change my status to head of household mid year?

I have been the primary caretaker this year and last year and paying a majority of the cost for medical needs, food, etc. I normally put single on my W-4 but I think I should change it to HOA. Is it okay to do that? Especially mid year. I was going to change my dependent status from 0 to 1 till I looked up what HOA is and it has me curious.