If you have water circulating through your sump it's just a matter of what is easier with the room you have.
I mix up some new salt water a day or two in advance, outside in a brute and fill my 3g water jugs. You can use any size you want, it's just easier for me to handle 3g vs 5.
I use my 12g top-off container filled with the new salt water, already connected to my ATO unit. I siphon water out from the display tank with a 1/2" tube covered with a screen. I am fortunate to have a drain built into my tank access panel behind the tank. The lost water is replaced with the ATO system. For my 120g display and 30g sump, I do this until I have emptied the top off container as many times as I wanted to, usually 2 or 3 times for a 24-30g change. It helps to have a buddy topping off the salt water so I don't have to stop the siphon or make a mess. Once I've replaced all the salt water, I refill my top off container with RODI to continue evap replacement.