and if you want to change back to the other, then you have to erase the device and start over.

You're not required to erase your device, but it's best to use it with a new set of seed phrases ^{[and a passphrase]}.

in the older devices it is firm ware, but the firmware can still be changed back and forth, no?

Yes, and it takes around a minute to complete: Firmware switching

The process for installing Bitcoin-only firmware is different from installing regular firmware. You must first download the firmware only for Bitcoin to your computer, here are the installation instructions:
https://blog.trezor.io/sources-say-orange-coin-good-new-bitcoin-only-firmware-now-available-47be0b611a4d

Trezor released two different cases. The universal model is black, front and back.

They released four different backplates... Apart from the cosmic black, you can choose the stellar silver and solar gold as well.