When linking a JIRA issue to an existing Portfolio issue, an initial synchronisation between the two items should happen in the following way:

Empty values for title/description/estimates in Portfolio should be filled with available values in the respective fields of the linked JIRA issue and vice versa. In case there is values in both, the JIRA issue and the Portfolio work item, either no synchronisation should happen or the user should be able to configure in which direction the sync should happen.