I think i got it does this seem right?
The solubility will increase. This is becuase HCl is an acid. So adding an acid to KHP solution will remove a portion of the base (HP-). Le Chatelier's principle states that the dissolving reaction of KHP will shift to the right, and cause more KHP to be dissolved, therefore increasing the solubility. However, the Ksp remains the same. This is because Ksp is a property of a substance, and it only changes with temperature.