Hello Hippy,
what version ePO you have? If you have version ePO 4.6, you can use export all tasks via this method "System Tree > Assigned Client Tasks > Actions > Export All Assignment" and after import to the another ePO 4.6
Unfortunately if you have ePO 4.5 or lower so I'm afraid you'll have to rewrite all the exceptions manually.
Good luck 🙂
Michal