You don't need to delete or re-add - both the old and new are options within the Sun and within the Sky. So you can just switch to whichever you prefer at any time.
EDIT - as a note, that's totally different from the portals issue, which is solved with the new Adaptive Environment Solver. Sun and Sky don't play a role in whether portals are needed or not. You can't swap back to using portals in Corona 6, they have been removed entirely.