FAIR WARNINGS:
1) yes, the unit is easy to pry open, but you WILL destroy the ‘catches’, that you need to close the unit afterwards.
2) do NOT attempt to remove the audio connector from the PCB; the connector is of the utmost flimsiest kind, and you WILL rip off the thin wires (ask. me how I know)
Now, I’ve ordered 1200mAh batteries and will install them instead of the 800mAh ones - will post back how that went.
In the meantime, it took me half an hour to pry out the audio connector, and reconnect the wires to them - under a digital microscope and working on a 24” monitor. Most people will not have this equipment, so think before you start. As for closing the unit afterwards, a few very small drops of superglue will probably do the trick. But I will see if I can open the second unit without tearing off anything; if I succeed I will post back also.
UPDATE: I managed 1) to open the second unit without damaging the cathches and 2) install the 1200mAh batteries. I just used the units for more than 3 hours now, and the indicators are still green. Great success (the old ones lasted half an hour)!