not uncommon to have to 'break' an impeller to get it off the shaft, most techs carry an 'impeller wrench' that allows them to easily grab the shaft of the motor at the back and spin off the impeller or in this case break off the impeller (keep in mind reverse threads) the new impeller should slide onto the shaft with the new Shaft Seal installed and thread into place, some also have a nut to tighten it down but it has to fit over the shaft, if yours does not then it is the wrong impeller