changing out tanks

[Pan]

Hello people,

I have a 20 gallon tank....that "I" will classify a reef tank, since it has some mushrooms, xenia, and a galaxea..
I Want to move the contents into a 20gallon long tank, better for light etc. Then use this 20 gallon for a sump for the new tank. My question is this, the tank has been running for 3 months now...well maybe a little more. I have 2 true percs, various inverts, 1 rainford goby. What is the best way to go about moving from one tank into the other....will i kick up ammonia from the substrate?
will bacteria die when i transfer etc etc...
Any ideas?