A 9.9 HP is a 9.9 HP how can you bump more HP to it. You can check if 15 HP has same CC compared to a 9.9, if so, probably will neeed to change carb, make timing adjustments. If not, start saving again for a larger HP engine.
That's probably same detuned 15 HP model. A matter to check with parts manual for parts compatibility between both HP models.
BTW, whay do you need more power, poor planing issues, too large family, overall poor water performance ? Which type pf boat is it ?