export/import of deltas (xml)

I would like to have the ability to export/import of deltas ( changes since specific date till now )
The situation is like this:
i am at a customer who wish to upgrade , and while upgrading , do some modification to his workflows , permission and screens.
so first of all i installed the new version , import current data from old jira , and started working on the modification
making sure everything is working and according to his specification .
Now that its all ready , and we want to go live , i wish to import only the new data that was created during my work
but i cant
i need to stop the old jira usage , export , then import and loose most of my changes
and redo all of them before i can go live again

or maybe i missed something?