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.