depends on whether the LR and sand are re-used....
are u reusing your sea water, sand and LR, if yes, then dump everything in 1 shot....don't waste time....
if u are getting more LR, as u are upgrading....ensure that the new LR are fully cured...if they are cured then dump everything in 1 shot.....the bacteria on the old LR and sand is enuf....
if u are using new sand + old LRs + new LRs, cure the new LRs, cycle the new sand...
stick to these rules
1) uncured LRs, cure them
2) cycle all new sand and LRs, if u are not using your old sand
3) try to use as much of the old sewater, sand and LR as possible and the filters media
4) get a bottle of beneficial bacteria to speed up the cycle
5) MUST NOT leave your LS in a bare tank (without LR and sand)