In a 10g tank it probably doesn't matter if you remove all the sand and replace it with new. In a tank that small the sand is only for your viewing pleasure and you are really getting no benefit for the tank from it. You may want to consider just going bare bottom but it is up to you.
Again, in a 10g tank it probably doesn't matter if you put the sand in first and then the rock. I prefer to have the rock sitting directly on the glass bottom of the tank and using the sand to actually stabilize the rock a little - you can always use a turkey baster to blow any sand off the rock that settles there. I only have sand in one tank now and am going to probably remove it within the next year or so. It is in a 230g tank so I will probably remove it in 3 or 4 batches.
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