Regardless what you modify, or how much, your code is derivative and not covered under fair use. If it contains even a smidgen of the original code it is prima facie copyright (and possibly patent) infringement.

Your option, if you intend to invent a better mousetrap is to study the application, not the code. What does the app do? What would you make it do if you could? Build your own apps if you want to steer clear of dangerous waters.

If the original code is open source then there must be a community of programmers involved. Join it and contribute your own add-ons, enhancements or suggested fixes. This way you will be doing everyone a favor, and get the credit due you for your contributions.