If you change an epic project to another then at the time of change, you can select if you want to move associated User Stories to new project or not.

• To Change an epic project, Open Epic Details page and change its Project field value.
• You can choose to move associated User Stories to the other project or keep them in the same project.

Note: If you move an epic with only selective user stories then remaining user stories will stay in the source project and will not be associated to the epic.