Dalton,
Yes you have to drill new holes,there is only one of the present holes that will not need to be filled and that is the center hole,all other 6 of the old holes will have to be filled but if done correctly there is no problem and also remember that you will not see any of them as they will be covered by the backing plate,all that needs to be done,is fill them correctly,when going to 12 x 18's you get to keep the same actuators you have on now so basicly you will only change baldes.
Others here recomend going to larger tabs/different brand,but as you my boat was brand new and I did not want to cover/seal any holes that would show thereby looking ugly,this worked out greta and boat handles much better in chop as I can get the bow down to fight any waves.
Robert Cibran