I've worked on large, old codebases before, too, and in my experience half the functions are at least partially incorrect (i.e., have bugs), too. The solution

I've worked on large, old codebases before, too, and in my experience half the functions are at least partially incorrect (i.e., have bugs), too. The solution in both cases is the same: when you find problems, fix them. Don't throw out the baby with the bathwater.