Can you do it yourself? Yes (probably). Should you do it yourself? Maybe, if you have the skills and know what you are doing.

The problem from the manufacture's perspective is probably liability if the user screws up the battery replacement in a critical unit like the PLB, and someone dies as a result of a failed battery replacement. They then get sued over a faulty design that allowed the user to screw up battery replacement.

In a device like a PLB, as an attorney, I would advise the Company to NOT design to allow users to easily change batteries, say like in a flashlight, and to make sure that a certified service is available to change the batteries. IMO, the legal risk of not doing so would be too high.