Not at all. What's stopping me from hashing the result with something else? The base of the work must be in changeable. That's the whole point of POW in this ca

Not at all. What's stopping me from hashing the result with something else? The base of the work must be in changeable. That's the whole point of POW in this case. If you want to rewrite history you need to do the POW again, in your system that doesn't need to happen.