Watermelon All of the other keys including the one for verifying OS updates can be rotated but not the verified boot key because it's flashed to the secure element while unlocked and enforced from there. It's also taken into account as an input for disk encryption and the hardware keystores so changing it would require wiping. Google cannot rotate their OS verified boot key either since changing the hard-wired key in the code won't avoid it breaking the other things based on it.
We generate separate keys for each device model so the keys do get rotated across generations of devices. Google does either the same thing for the stock OS and firmware. They used to share keys across models launched at the same time. These were the shared stock OS signing keys for 3rd gen Pixels onwards which likely also applied to firmware:
- Pixel 3 and Pixel 3 XL
- Pixel 3a and Pixel 3a XL
- Pixel 4 and Pixel 4 XL
- Pixel 4a (5G) and Pixel 5
They haven't shared keys within the same generation after the Pixel 4a (5G) and Pixel 5. It's a bad practice since it means components from one could be used on the other despite not necessarily being compatible and potentially even creating a security weakness although not in a way that's likely to matter much.