When changing the tire, you should replace the stem...the actual sensor screws or bolts to the stem, and they are meant to be easily separated. The batteries are not replaceable (as far as I've seen), so when the sensor eventually dies, you'll need new ones. The reason to replace the stem is that the O-rings that create the seal can and do get hard and can crack...a new stem gives you a new valve and seals. Also, the stem can become corroded, and begin to look bad. As an aside, you can get new stems in various finishes and colors, so you could swap them to complement or update the look (say in chrome or black, or red, for example).
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)