I would not attempt to remove the SP-6 valve on one drum while operating the other drum in the pair. Cokers use double block with steam pressure above process pressure in-between the two valves to provide isolation for routine coke removal. Removing the SP-6 valve for maintenance requires positive isolation. Using the swithch valve only with the unit still operating has very high risks of hot hydrocarbons being released to the atmosphere. I would not remove the SP-6 valve if the unit was in hot circulation either.
To remove the SP-6 valve, my recommendation is to remove hydrocarbon from the heater and purge the heater and the transfer line with steam. You can then depressure and isolate the feed system and then perform maintenance on the SP-6 valve.