On my 2011 MBP I upgraded from Mountain Lion to Yosemite with no issues on my boot camp install running Windows 7. However I admit to hesitating on the Sierra upgrade. My impression is that the Windows partition is self contained for most purposes, and that it should continue work, but I'd research some more before leaping. A quick google search found a discussion about upgrading to Yosemite (no problem), but I found none about upgrading to Sierra.