"For sure buy the kit to change the watch battery"
Most definatley buy this kit! If you have a 6 series watch(I have the T6) you will want this kit to change your battery. it comes with a special tool for removing the battery case lid and a new seal and cap. You may be able to save a few dollars on the hrm monitor by buying just the battery and reusing the seal, but DO NOT DO SO WITH THE WATCH. BUY THE KIT.