Switching after 2 years is not impossible, if you'll be happier as a bio major, then go for it. I was even thinking about switching after 3 years, but at that point, it's way too late to catch up in a reasonable amount of time.
In the end, just do what makes you happy. An extra 1-2 years is nothing compared to the rest of your life spent doing something you don't like...
The Cooper Union
Mechanical Engineering 2012