Yep, all of it. I bought new to top it up for the upgrade. I put in new sand, as transferring sand can cause a huge spike (it's impossible to shift it without crushing the tiny critters living in it). An alternative would be to wash the old sand, then add it back in. You could even start removing a little at a time and preparing it beforehand (not all at once, as a tonne of bacteria will be living on the sand, so you don't want to remove it all in one go and find the bacteria in the rock can't keep up).