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You would be better off changing out the cc for the sand over a couple of weeks.
Dumping the sand on top would result in both trapped detritus and a smothering of bacteria. Even though the sand is live it won't have nearly enough bacteria to prevent an ammonia spike so it's best to only change half or third at a time.
THe CC will also over time filter to the top as the sand sifts to the bottom. Clearly its more work to remove the CC but its worht it in the long run. My first tank I started with CC. Once I started to learn more about salt tanks I realized sand was what I shoudl ahve done and I did the swap out just like my article in the article area. If you follow those steps you shouldnt notice any ill effects.