i transfered my 1ft cube's sand to my 2.5ft... but even with that little sand the detruis from the old sand was insane... clouded my whole take for almost 2 days...
My advice is to get holding tanks/pails, put all ur lifestock into them with all the essentials, bubbler, small light maybe... then transfer all your old livesand FIRST... why I say first, is because you don't want to pour dirty sand into an already filled tank...
what I think I did was to lay down some new sand, then some old sand, then some new sand... etc etc.. and then mixed them around then added new saltwater... then the rocks, then the livestock last... coz I din have much livestock to begin with from my 1ft cube... but for you maybe best to wait a while for the tank to settle...
I also ran filterfloss for the first part of the tank to trap all the floating detrius...