Yes, but console vendors generally prefer not to allow downgrades.
So if v1 is signed by key A, v2 is signed by key B and invalidates key A; a console that installs v2 wouldn't be able to install v1 after, but that's not a problem for Sony.
But, I'm not sure how many companies would be able to manage their keys properly to ensure that someone with access to key A doesn't have access to key B.
If these are asymmetric key pairs and the device side key was extracted from the device... Switching keys wouldn't help, and it's not a huge deal by itself --- having the device side key doesn't allow you to make a firmware image the device would accept.
So if v1 is signed by key A, v2 is signed by key B and invalidates key A; a console that installs v2 wouldn't be able to install v1 after, but that's not a problem for Sony.
But, I'm not sure how many companies would be able to manage their keys properly to ensure that someone with access to key A doesn't have access to key B.
If these are asymmetric key pairs and the device side key was extracted from the device... Switching keys wouldn't help, and it's not a huge deal by itself --- having the device side key doesn't allow you to make a firmware image the device would accept.