You could definitely use the old filter media in the new set up, just place the media into the magnums, and run the filter media with the new media you have for the other magnum.
All you need to do is get the new tank set up, gravel, decor, plants , water in and set the temperature at the same as your old tank. Leave this running for a day to get the water up to the right temperature (I would even stick half the media from the HOB into the magnums to get it kick started).
The next day, flip all your live media into the magnums, and as long empty the majority of the old tank water into the new tank. This will lower the amount of water in the old tank, doing 2 things,
1) making the water level lower so you can catch those crafty fish!
2) Add some of the same water into the new tank so give it (albeit) a small amount of bacteria that is present in the water column and even out the water chemistry.
then gently add the fish into the new tank (As long as their similar temperatures)
Your water will be the same as the old tank as you get it from the same source (i.e. tap), it would be a different story if you was to move locations and kH, pH were greatly different,
I have used this method going from 60l to 100litre, then 100l into my 400l and everything has gone smoothly