this would be my worry too

the forces on the old frp just forward of the beam by the leverage of the new bows would considerable

the major failure mode of long bowed old nacras and supercats seems to be 1 bow failing at the beam while on heavy seas or 2 on trap and flying a hull

i would think you would need 2 or 3 more layers of cloth forward of the beam for strength and possible a bridal foil like 90's nacras