No need to use the old water. Add fresh tap water to your tank adjusted for temperature and add dechlor. 5-Gallon buckets are the simplest ways to move discus. I've put discus into 5-gallon tanks and driven them 12 hours to a NADA show for competition with no negative results. You don't need anything else in the tank - no heater, no air. They'll do just fine.
1. Move your discus into 5-gallon buckets with existing water. Important to have lids.
2. Dismantle your setup, do a thorough job of cleaning out the gravel and tank surfaces
3. Move the setup, set it up, refill, add dechlor
4. Put the filters back on the tank
5. Put the discus back