no need to run filter for 24 hours, you can use the water from your 10g. (in fact, if you DO move your filter to the new tank for 24hours it'll be not only bad for your fish in the 10g, but much worse is you'll kill off the beneficial bacteria on your filter media.
so just do the following, real easy.)
shut off your filter, heater and all hardware. siphon a little bit of water into a bucket, and stick your filter media in there to keep it wet. (only need enough water to cover the filter media, it'll only be in there half hour tops.)
put your 20 down on the ground.... siphon about half the water in your 10g (5gallons) into the 20g.... then go ahead and net out your fish and put them in the bare 20g that's now 20-25% full. (may have to remove deco first to make it easier to catch them, just stick the deco in the bucket with the little bit of tank water to keep everything wet.) both tanks should be easy enough to lift at this point without too much stress on the tanks. get someone to help you with the 20g maybe just cause it's probably longer.
remove all the hardware from the 10g, and stick the 10 on the ground or on a table next to wherever the 20g is going. once the 20g is where it's gonna be you can set the hardware up on it, but don't turn it on yet.
get as much water out of the 10 as you can and put it into the 20g. then move your gravel/substrate over.. just take your time, so as to not freak out the fishes. now top off the 20g with treated tap water until the water level is high enough to cover your heater and to start your filter up.
add deco to 20g.
top off again with water so it's full.
you're done.
all you're changing is the container.
i recently switched a 10g over to a 15g just like this. (too bad my 15g leaked, so triple check that 20g first!)
