Just the EOG car has to be modified. While the terms are very frequently interchanged, usually what a HOG system requires is termed a 'Power car'. This just contains the step down transformer and other control systems. An EOG car is basically a power car with an integrated diesel generator. Since an EOG car is still required as a backup unit even if trains are running with HOG, instead of manufacturing separate power cars, IR is just modifying the EOG cars so that they can function as either a power car or an EOG. Most probably this is what they meant when they said 'non-modification of rake'.