On some helicopters the engines are capable of exceeding the transmission limits in certain conditions. For example, on the SK-76 A models I used to fly you had to be aware of both sets of limits; obviously the first limits you meet have to be respected.
In a case like this, the main gearbox and other components need to be upgraded if more powerful engines are installed.
As per the "B" model, which had more powerful Pratt and Whitneys than the "A" and had a more robust gearbox (a thirsty machine). There was then the C model, with smaller engines for economy, coupled with the B gearbox. And commonly known as the "B minus".... at least you only had the engine limits to worry about in those.