First of all, do not cheap out and buy your replacement batteries from eBay. Instead, buy them from someone reputable, like Digikey. I tried the cheap route and despite the batteries arriving with 3.27 volts, they don’t pack enough amperage to run the Mazda keys when they get below 3.07 volts.

Battery sourcing aside, this YouTube video does a good job of showing how to take a Mazda3 key apart. If you happen to snap any parts off and the keys don’t want to stay together, just dab some super glue blots, one each corner, and let the key dry. It will still come apart the next time you need to replace batteries.

When it comes to reprogramming, this guide may be helpful.