How is it more difficult to rewrite history in pow? Maybe only if you want to stick to the same exact algorithm going forward. But a modified node can do whatev

How is it more difficult to rewrite history in pow? Maybe only if you want to stick to the same exact algorithm going forward. But a modified node can do whatever it wants, including rewriting the blockchain from the beginning to use a different hashing scheme, making custom exceptions for individual block numbers, etc. It all depends on how the modifications align with the incentives of the coalition and user base.