You don't need undergraduate courses. I am an environmental sociologist who did not take a single sociology course until graduate school. Your environmental science background will extremely useful. Just make sure you're applying to programs with a strong environmental focus, try to figure out what the hell is going on in Soc as fast as you can (you may feel like you're behind in Theory for a long time, but remember that "being well read" is an endless ladder and grad school is the best place to build that foundation), and try your best to articulate to the program in your application why you want/need a Sociology MA/PhD to create the career/research you're looking toward. If you have the opportunity to teach, it will also help immensely with developing the fundamentals.

Sociology needs more people trained in Environmental Science! Today's environmental problems are extremely urgent, and their social components require much greater description!