Modified?
well they may have to meet a stricter time frame on updated service bulletins.
Basically they are tracked against their hard shutdown rate as a fleet and their maintenance is scheduled against this tracking. In some cases this might mean prioritizing an updated service bulletin on this fleet vs a quad fleet. In the long run its the operators call (he has to demonstrate to his authorities that his reliability meets ETOPs standards).
However in some cases the data may be more compelling (a high fleet failure rate of a component) and require a fleetwide restiction of ETOPS until a retrofit can take place.