In short, I would say that it's a no-go. You have to destroy the casing to get at the battery which is soldered onto the PCB and then smothered in hard resin. I went in from the back of the sensor, suspecting that the battery would be just under the rubbery resin. It wasn't, the PCB was, and the battery is soldered to the other side of this PCB. I prised the PCB out, destroying it and ripping the tracks where the battery is soldered.
In this photo you can see the battery on the left under a white plastic cover that lets the contacts protrude through. These contacts are soldered onto the PCB on the right. The PCB is as it came off the sensor, so the contact points avec the battery are hidden.
With the PCB flipped over you can see circled where the battery was soldered:
And now with the battery removed. The resin (brown) around the sensor and on the outside of the sensor is quite rubbery and easy to remove. The resin (white) holding the battery in place is much harder, but like compressed powder. I eventually prised the battery out of it, damaging its case:
I had a go getting at the battery from the other side by cutting the case with my dremel. I could get the battery out, but not without destroying the PCB.
So in conclusion, I think it's not a practical proposition to try and change the batteries in these things.
