Correction: pay $400/hr to gain a temporary majority in the block mining lottery. You can do a variety of things with that majority power, but “rewriting history” is a bit misleading. Yes, you can probably get the network to discard some previously accepted blocks, but this only allows you to invalidate your own transactions plus the transactions that depend on them.

Furthermore, any blocks that were mined under high hashpower still need the same hashpower to be re-mined. A one-time reduction in network hash power doesn’t make previously mined blocks easier to mine.