Pass only the Board object and do a retrieve there of the userstories.
Regards,
Ronald
[EDIT]
You need to use a helper object because you need to remember the current list of stories attached to Board. So create an helper object and attach the current stories. Show that object and let the user add or remove stories to that list. Place a confirm button to process the list. In that microflow you can then compare each object if it was added, removed or not changed and do the stuff you want to do based on that. There are some list function to do these kind of checks.