The protocol https://en.bitcoin.it/wiki/Protocol_specification includes a lock_time field for transaction (bitcoin doesn't currently implement it), the description from the wiki: "The block number or timestamp at which this transaction is locked, or 0 if the transaction is always locked. A non-locked transaction must not be included in blocks, and it can be modified by broadcasting a new version before the time has expired"  Then in TXIn there is a field for sequence, describe as: "Transaction version as defined by the sender. Intended for "replacement" of transactions when information is updated before inclusion into a block."

What is unclear to me is what changes to a transaction would be acceptable in an update, could you change the source coins, the destination address, the script?  And, if they are changed, how would miners determine that this is an updated transaction and not a new, totally different one?