Is it true that batteries cannot be changed on the estimote beacons?
Yes, that's true.
Inside each Beacon is a non-rechargeable lithium battery. A beacon’s lifetime depends on its configuration. At default factory settings, Estimote Beacons last for about 2.5 - 3 years. You can extend the lifetime to over 4 years by activating power modes, more informtion here: https://community.estimote.com/hc/en-us/articles/202664276-Check-out-new-Estimote-power-modes-for-your-beacons