It's always difficult to answer questions like this, because code can be structured in so many different ways, good and bad, to do the same things. It's probably safe to say in this case that for a well-written body of code this would be a simple change: likely changing the default value of a flag, or the boolean expression in a branch. However the code could be written to make it much more difficult, and there is just no way to get a sense of that without understanding how it is put together.
