Can I use existing 40 AMP breakers when the item calls for 30 AMP
It may be possible if the new water heater doesn't state the maximum breaker size.
If the manufacturer only states the minimum, since you are not at 150% I would hook a triple 40a to the new triple 30 water heater.
It is not Code compliant to go the other way from 3 30 with #10 to a 40 this is correct.