Bose QC35 IIs – replace the battery?

My Bose QuietComfort 35 IIs are about 3 years old now, and the battery is starting to get rubbish. I get about an hour if using noise-cancelling and the microphone, 3 hours for audio only.

I contacted Bose to ask if they service them, but the answer is no. They offered a replacement I could buy (with a discount, but still more expensive than most places are selling them anyway).

But I really object to buying a new set of headphones when these ones just need a new battery.

Has anyone tried replacing the battery? I've seen a few videos, but it seems quite difficult. Since I can still use them with a cable, I'm worried about destroying them entirely.