I empty my old soil into my compost tumblers but I do only use organic feeds nowadays. If you are using chemical nutrients then I would not recommend doing this as the salts will kill off your microbes in the compost.
I don't recommend it. Reused soil/medium can hold pests, insects that could bring problems that you don't need. I tried it and had problems with mites and other pests.
Best to put the old soil into the outside garden or elsewhere and use new soil / medium for the cannabis grow.