If the media is in there for 3 weeks you should be good to go when your tank tips up, just feed lightly and monitor the water quality closely.
If it were me, I'd set the filters you are going to run on the new tank, on the one you already have running at the moment. This will give them a jump start on the cycle of the new tank when ready in a few weeks.
And then as soon as you get the new tank filled and ready, transfer them to the new tank.
You could also put any decorations you are going to put in the new tank, in the old one to get some biofilm going.