In Med Rad Sci, you already take bio 1A03, Math 1LS3, and two kin courses which are all Life sci courses. In addition, you could take 2 electives as Chem and Psych, which are also life sci courses. So even if you go into Med Rad Sci and want to switch out next year to Life Sci, there's not much catching up to do (you could also go to summer school). I know this because I'm in MedRad and I thought about switching to Life Sci because i wanted to go into pharmacy (but later I decided not to do). And I found out that it's actually very easy to switch programs from MedRad to Lifesci as long as you talk with the academic advisor before June (when the course selection begins). But i think it's hard to change from Life Sci to MedRad. So i recommend trying out MedRad first (first year is pretty easy).
I hope that helps