As Wolfshadw said, the purpose of new firmware for devices is to fix current functionality or add new functionality, not generally to make things worse, however, and Hollywood does reserve the right to, with new firmware, make current movies unplayable, and players useless.
On the plus side, Hollywood has never chosen to implement that feature to the best of my knowledge. From what I've read, the same feature is built into DVD players, but has remained unused. It is hard for Hollywood to sell a self-destruct equipped movie, as we saw with the original, failed Divx format, so the easiest thing to do is not tell customers about it. They claim the reason for the need to disable movies or players is for the case that a particular movie was compromised and the encryption no longer good, they could reissue the movie with new encryption and disable the older one.
As for reversing firmware, it depends. If the manufacturer allows you to install older firmware on a device, you can, otherwise no. Generally there is no need of this. What I was getting at is, if somewhere during the firmware process, which on Blu-ray players can take up to a half-hour, your device is interrupted, perhaps by power outage or a child coming and playing with the device, since you will likely end up with incomplete firmware on your player, your player will no longer function, and in all likelihood be unable to install any firmware at that point to fix the problem. It is a process you need to do when you can be reasonably assured there will be no interruptions of the player.