can i transfer mid semester from science/commerce to biomed/commerce and if so how hard is it to do so? can i get full credit?
You probably aren't able to do a mid-year transfer (I assume that's what you meant by "mid semester"?) because of biomed's course structure which consists of sequential core units, but to be sure you'd have to contact the faculty.
That said, you can absolutely transfer at the end of the year. You won't get any credit for your science units if you're aiming for one of the 50 guaranteed postgrad med places (as you need to complete every BMS unit), but you can certainly transfer credit over for commerce.
Because of the huge increase in demand for places in biomed over the past couple of years, I'd imagine that you'd need an average of 80+ to be truly competitive. There's probably a minimum WAM of 70 to have your transfer application considered by the faculty, but again you'd want to check this with the faculty. Good luck!