From what I can ascertain, the new bigger battery is incompatible with the old chassis: https://frame.work/products/pro-battery-74wh
"This product can only be used with both the Framework Laptop 13 Pro Bottom Cover and Framework Laptop 13 Pro Input Cover."
I applaud that the mainboard and keyboard are backwards compatible, but I don't think the pro is quite as backwards compatible as some are thinking
They actually call out this specific incompatibility in the video here (at 2m44s): https://www.youtube.com/watch?v=JSxgCEpkiKM&t=2m44s
Mainly just a trade-off because of because of the bigger battery (but it sounds like they'll sell you the bottom chassis pieces as well).
Yup. Sounds right. Interestingly though, you can use the new input cover with the older frames.