Tbh I don't think there's much of a difference between HL and OL biology... It's just the marking schemes that are different..
OL are more lenient I think
I'd stay with HL if I were you but it's your call
My teacher told us most students go up about 10% in June compared to the mock, but I'd say do a few past exam papers within the 3 hours without notes and correct them yourself to see where you're at, it'll be a good indicator as well as good practise, and then you can decide yourself what level will suit you best. Good luck!