Different pieces of software from the same code base but with different feature content should have different version numbers in some way - so you need to change something in the version number (or product name).
Do you plan to submit the first change back to the project? (You probably should.)
Is the second feature, the hack, one that you will improve over time? You might keep it on your own development branch so it is easier to maintain separately while still importing updates from the main project.
Or are you planning to stay separate from the main project in perpetuity? In that case, you should consider renaming the software as well as changing the version - or somehow making it clear that the version is yours and not theirs.