Thanks for the guide, I've just successfully replaced the batteries in both my earpieces:
- To heat it, I grabbed the earpiece on the pipe end and held it in a thin plastic bag, using it as kind of a glove to keep it dry. Then I submerged it in hot water (~95°C) and wiggled a bit for better heat transfer. Be careful to not burn your fingers. After about 5-10 Seconds the earpiece was nice and warm.
- Then I used @ws6transam's tip, wrapping a small pipe wrench with some bike tube rubber: I set the wrench to almost to the biggest width of the earpiece at the seam and gave it a gentle squeeze while it was warm, re-heating it as needed, until i was able to fit a plastic pick into the case. As you can see in step 4's picture, the seam has a ridge on the "earwards" part of the earpiece, so insert the pick's tip/edge pointing to the top shell...

- I skipped removing the ZIF connector cable and carefully unstuck the battery using a pick to lift and the halberd spudger to unstick the glue.
- Afterwards I did the Re-Init Procedure