Ok. Since you have time, another option would be to try to find a fellow reefer (or an LFS that sells used tanks) and borrow a large tank (90-125 gallons), set it up with HOB filter and skimmer, and move the fish there temporarily. This way you can feel more comfortable about keeping the fish in holding and take your time with the tank swap.
Yes, this would be an additional move, but since you have to set the new tank up where the old one is, there's not a scenario where you can guarantee a quick swap... something is bound to go off track at least a little, whatever plan you come up with.