The easiest way to switch it over would be to take your uncleaned filter from the 30 gallon and tank rinsed gravel and add it to the 100 gallon. The gravel part may not even be needed.
You will only lose some bacteria, from the 30g tank glass. Your filter basically has your bioload that you are transferring, and the 100 gallon will dilute the ammonia produced a lot with the current load of fish.
If you have a bio-wheel filter, or a filter packed with bio-media, you don't even need the gravel from the old tank.
I would setup the 100g,dechlorinate it, and I would switch the next day. That way your temp is the same, the Oxygen bubbles are mostly gone that a newly filled tank has, that can stick to the fish(maybe bothering them), and you can let the dust settle from the new gravel you are putting in.
I am getting excited thinking of your switch, I too may be doing this soon!