This is not true. Bitcoin-esque blockchains are NOT append only. The only thing clients do is: 1) ensure the blockchain they have received is valid, and 2) that it is longer (more total PoW). If those conditions are met, they will consider that new chain the current chain.
You can come up with a new, completely different sequence of blocks, send it to clients on the network, and get them to start using that new chain instead as long as it has more PoW.