You can do a zfs send/zfs recv to send a dataset (including all snapshots) to yourself which is effectively rewriting the whole dataset, including all of its hi

You can do a zfs send/zfs recv to send a dataset (including all snapshots) to yourself which is effectively rewriting the whole dataset, including all of its history, by duplicating it. Not hard, but it does require sufficient free space. Once it's done you can destroy the original dataset and reclaim the space.