To be clear, a badly-programmed AI may modify its own goals. People here usually say "paperclip maximiser" to indicate an AI that is well programmed to maximise paper clips, not any old piece of code that someone threw together with the vague idea of getting some paperclips out of it. However, if an AI is built with goal X, and it self-modifies to have goal Y, then clearly it was not a well-designed X-maximiser.