Hey,
About a year and a half ago I was a physics graduate student and I decided to switch to medicine. Other than getting the prereq's done, it has not been too hard. I did get lucky in that I was allowed to stay on as a TA and take my classes for free, but if you are still in undergrad there is time to switch. I was a little short on community service due to my physics track, but I worked really hard for the last year to volunteer and the schools have respected that. As far as your question goes, there are tons of concepts in biology and I have found my physics background to be immensly helpful. It is especially helpful on the mcat, b/c you should do great on physical sciences. If you really want it, I say go for it but you have to go all in with volunteering/clinical stuff/etc. if you want them to take you seriously.