They put a headrest cover on the headrest. This makes it easy to convert seats between J/Y. The biggest service difference is J gets newspapers, Y does not (even if they are left over). You also get first choice on the food as it goes by (the meal service is sort of like tapas, they have lots of different little dishes, some quite good). There may also be a change in seat pitch, with the rear seats closer together, but much of Y is exactly the same as J on these planes, all are quite comfortable. I can't over-emphasize how much better it is than IB mainline in Europe.