Your unassigned seating venue should still have seats in it, which means you can allocate those seats in many of the same ways you would for an assigned seating venue.
For example, if you want to do a promo code-based mode of sale switch, you could allocate your limited premium seats to that mode of sale and make the allocation inaccessible from the regular modes of sale.
Additionally, even if a perf is already on sale, it is possible to add new price zones if that is necessary for your premium offer. Just add the zone to the map in use by your performance, but don't alter the default zoning for your screens — just edit the zoning for the performance in question using season maintenance.