Magic Bands operate using RFID chips. There are two of them in each Band. One is a passive chip - it does not have it's own power supply, but instead relies on a minute amount of power that it gets whenever it touches one of the Mickey Heads. This chip should last virtually forever, provided the Band is not damaged in some way. The passive chip is used for everything that really matters: opening the door to your room, making purchases, entering a park, entering the FP line, etc.
The second RFID chip is an active chip and has it's own battery. This chip has a short-range transceiver in it, and it is that which is powered by the battery. This chip is used for get some ride photos (7DMT & PotC for example), as well as the goodbye messages that appear in IASW.
Disney has said from the beginning that battery life was 2-4 years and anecdotal evidence, including my own, seems to confirm that. Most Bands will still have some battery capacity at three years, but YMMV. But again, for all the critical stuff, a Magic Band will last until the band is damaged, Disney changes the underlying technology or they do away with the entire concept completely.
Your old bands will still work for everything that is important. There is no way to tell if the battery for the active chip still has a charge.
Steve