You don't need to change the validation rules to double spend. What you do is this: - spend btc with a transaction and get it included in the chain. - start

You don't need to change the validation rules to double spend. What you do is this: - spend btc with a transaction and get it included in the chain. - start mining blocks starting from before your transaction in secret - include a trnasaction to a different address in the secret chain - broadcast the secret chain once it's longer than the "official" one - since your chain is longer, your chain is now the official one. All the transactions in the other chain are now discarded