No problems with ride retrofits in Ohio. From an observational standpoint, one needs only look at the Blue Streak (major retrofits in 1994 and in 1996) at Cedar Point, or more dramatically, at the Racer, The Beast, The Beastie, and most obviously, Flight of Fear at Kings Island.
Ohio law requires an annual inspection, and an additional inspection if the ride undergoes a major modification mid-season. In general, Ohio's inspections are to manufacturer's specifications (which makes me wonder how Kings Island's wood coasters are permitted to operate) and provisions exist for handling manufacturer-approved retrofits.
New Jersey also allows changes to the ride or trains, but any such change is required to undergo an engineering review with the State.
--Dave Althoff, Jr.