If a database snapshot and a checksum are published out to a CDN on a regular interval, you can certainly run UPDATE on a row and publish a new snapshot, but those tracking the integrity of the data will detect the change.
If you are in control of write access to the blockchain, you can roll back to whatever block suits you better, and just fork it. Others might protest, but those fools have no write access, so they're stuck with your fork for the time being.
Blockchain is just a protocol for consensus-based DB replication, you don't somehow magically end up with a superior DB because of that.
