How should you close a 24 ft diameter above-ground pool for the winter if you are going to convert it to salt water in the spring?

You should go ahead and close the pool as you normally would. Add only sufficient sanitizer to prevent growth. When you want to open the pool, contact me and I will be happy to tell you how to convert to the saline system. Note: If you are presently using 'baquacil' you will need to dump the water and refill before converting to salt in any case, so not so important to keep water perfect throughout closed down period.