No you can't do this. Disregard the DAO comments. Yes, a hack caused a hard fork - but the fork itself was not a hack. You can't update an account value withou

No you can't do this. Disregard the DAO comments. Yes, a hack caused a hard fork - but the fork itself was not a hack. You can't update an account value without issuing a valid transaction (requiring the private key). What you can do with a majority hash power is roll back transactions. But this is limited to your ability to generate a longer chain which becomes more improbable with each mined block. This attack vector isn't that great because transactions worth larger amounts will wait for more confirmations. You're essentially left with an expensive DoS.