The POPUP function was created in the original database which I migrated the Universe from. The Object is still pointing at the Original database but when I try to change it I have an error message.
Surely the POPUP function (if it is a function?) would have been migrated too with the import of the universe?
Database functions (or tables, views, stored procedures, database indices, etc.) are NEVER migrated with a universe. That is something one must take care of separately on the database side.