I think the problem here is that I didn't specify exactly what I meant by "refactoring", "unit" and what types of changes I would be making. Often times I find refactoring necessary because a significant piece of the system was either under or over engineered and it becomes necessary to update that code without changing the functioning of the system. So by refactoring, I mean refactoring at a level the encompasses multiple units of code, while maintaining the same functioning of the system. That means the interfaces may change for the individual units, which would obviously break the unit tests.