If someone tries to fake a transaction by changing a block that had already been stored in the blockchain, that block's hash would change. (kryptomoney.com)
«If I can persistently be in their network I can issue fake transactions.» (cnbc.com)
Well, if anybody tried to fake the transaction using an altered block that currently exists in the book chain, that block's hash would also alter. (livebitcoinnews.com)