Okay, hockey is over, I can pay attention now.
Let's start with getting the 40g up and running for the fish you currently have, and look at new stocking ideas after.
Depending on your substrate, filter, other details, I can't necessarily give you an accurate step-by-step specific to your setup, but here's a basic plan. Adjust as needed or let us know if you foresee any obstacles to these suggestions.
First, since you can't have both tanks running at the same time, you want to ensure your fish are as comfortable as possible while they are in transition. This means having a large container that they can be housed in, with their existing tank water, and some heat and aeration. Maybe you can put your heater in that container with them, along with a running airstone. Second, you want to preserve as much of the good stuff already in your 20g so you don't go through a cycle when you set up your 40g. That means having enough buckets to keep most if not all of the 20g's water. Plus you will want to move your existing gravel, full of the good bacteria, into the new tank without washing it or letting it dry completely. Also, don't rinse your filter media - it, too, is full of good bacteria.
Once the fish are out of the 20g, the filter is turned off and the heater moved to the fish bucket, and the tank water is reserved, move the 40g into position (you have cleaned it appropriately well before!!!) and scoop the wet gravel into it. Set up whatever decor you have and the filter, unplugged. Then add a bucket or two of the old water into the tank, and if you have live plants, arrange them. Then add another couple more buckets of the old water.
Meanwhile, you should have some tap water at the right temp (or higher) and treat with water conditioner like Prime. Add to the tank, and if water level is high enough, start the filter. Let run for a couple hours to get rid of the stirred up sediment. Then move the heater from the fish bucket to the tank, and maybe either keep the fish bucket well covered so the temp doesn't drop while the new tank water reaches the proper temp, or have put warmer water in the 40g to begin with - you don't want the fish to go through a massive temperature change. Finally, introduce your fish to the new tank with their fish bucket water, and if needed, top up with more treated water.
Now I need to go sulk because the Canucks lost. Good luck with your move and please don't hesitate to ask lots and lots of questions about anything here - this is a great forum with lots of experienced people who want to see you and your fish thrive!
