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?
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.