Why would they need to know? If you changed everything in the transaction, what difference does it make whether it's updated or new? And if, say, leave at least one of the coin sources the same, obviously only one transaction can go through anyway.
You want to be able to cancel the previous transaction. Otherwise, miners wouldn't know which one to include in the block chain.
You lock the transaction so it isn't included in the chain and until that time you can update the transaction.