Anything else in there? If not, just start using low brackish levels for water changes, gradually increasing the levels. Once the water is up to marine levels, just move the fish.
The water isn't really 'cycled'--it does not contain the beneficial bacteria. They are attached to the filter media, substrate, and decorations in the tank. As the salinity rises, the FW version will die off, but the SW type will develop. Done right, there really isn't a spike. Too fast, and there can be.