How do you convert a fresh water hot tub to a salt water hot tub?

Although ordering the spa as a "salt water" hot tub when you buy it is easier, as there is some Plumbing work required, you can retrofit either a bromine salt generator or chlorine salt generator and create your own "salt water spa".

There is a misnomer about salt water hot tubs in that you still have to create the chemical chlorine or bromine in order to sanitize the water. Only with a salt generator this process happens automatically, but you must establish either a bromine salt reserve or a chlorine salt reserve depending on the system you are using.

You will save the trip to the store for chlorine or bromine, but you still need to add the salt when you refill the spa or when you top up the water.

To convert, the bromine salt cell is plumbed into the water line and the control unit is mounted to the spa. A hot tub tech can do this for you. Sometimes you may save by already having the salt generator and just having someone hook it up and add it to the plumbing.
There is no switching. But you do have to install a chlorine generator. The generator makes the chlorine from the salt you add. But why go through all the hassles and expense?

A better bet would be to use Bromine, it was developed for use in hot tubs. Besides at high temps. chlorine will vent off in a gas form and probably run you off.