Good Day
During the EHP installation the SUM tool prompts you to release all the unreleased transport requests. Once they are released the cannot be modified anymore.
After the EHP upgrade if you create a transport request from a released transport with the transport of copies option and include the objects which are part of the old transport then the new transport request will have the latest version if there was a change in the version due to the upgrade.
This is my understanding. I don't think the system is extracting data from the data file of the transport request to create a new one. Instead it must be using the objects in the transport request as a reference to the objects in the SAP system and then creating a new transport request with the objects in the system.
Check these links:
http://help.sap.com/saphelp_nw04/helpdata/en/57/38e1a94eb711d182bf0000e829fbfe/frameset.htm
Regards
RB