This is discussed more than once, so a little search should answer your questions. Today, the boiler keeps the water hot enough to allow the system to heat your domestic hot water supply. You don't need it to stay that hot any longer once you reroute the water to the WH. But, depending on the design of the boiler, there may be a minimum temp it needs to run during the heating season (and you could probably shut it down until then) to prevent damage to the burner. If the water is allowed to get too cool, if the boiler isn't designed for it, you'll get condensation, and that can eat it up quickly.