Preventing forks from breaking away with a shared tx history prior to some block is impossible. If you're going to hardfork away from decred, then you can do anything you want to maintain the tx history. For instance, at block X empty the ticket pool, reset ticket price to 1 dcr, reset PoW diff and change algo, if necessary modify other checks to allow ticket pool to be emptied, etc. It's ludicrous to pretend that no one could hard fork decred and start a split chain with a shared tx history prior to some block. Would it be more of a pita than doing the same thing to Bitcoin? Sure. Would it be impossible? Controller nuclear fusion isn't impossible, this sure as shit isn't impossible.