The battery failure warning came up after 358 cycles with a max charge capacity now running at 3613 (Max rated is 6700). I would always need a power-supply nearby as I'd need to plug back in within an hour or so of running.
You need to press down firmly on the pentalobe to get it to turn as they are held in with Loc-Tite (blue). Once you break the seal, they all come out easily. I think the total time to replace was around 15 minutes? Really easy. I replace it with a A-Stock battery. It was around 95% of a new one. Should I have spent more on the "New" battery? Let's see how the life is.
No real tricks to this. Perhaps it would be smart to add loc-tite back to the screws so they don't work themselves out later.