Imagine that we have a 40-year-old automobile and decide to replace the engine, hoping to get another four decades of trouble-free operation. The flaw in our logic is that we have overlooked the other components of the system, such as transmission, electrical, exhaust, and body. These other components have the same mileage and amount of wear that the engine had. This analogy is similar to expecting the same results when replacing just the boiler on a project and ignoring the rest of the system.
Why are they replacing the boiler? I believe that we need to know why they are replacing the boiler because if we ignore the reason for the replacement, the new boiler could suffer the same fate. It is rare that someone replaces a boiler simply due to the age of the unit. It is usually for some other reason such as leaks, reliability, or parts availability.