As you probably the whole beauty of a DSB in a large reef tank is to grow anerobic nitrate reducing bacteria in the deeper anoxic areas of the deep sand bed. Something I now miss in my nano. Since the footprint of your sandbed is likely increasing you will need more sand. I believe the general recommendation is to add the new sand, then transfer the sand into the new tank. This will allow the new sand to become quickly repopulated with anaerobic bacteria and the mixture of aerobic and anaerobic bacteria will quickly settle out with the new oxygen gradient.
Unfortunately, some people do not recommend strictly transferring your livestock into the tank until your sandbed quickly recycles. I have also heard of livestock loss when you disrupt a DSB (i.e a powerhead falling and mixing up the sand). So if you follow all three recommendations, you will have to remove your corals to a third tank, transfer the sand and wait for it to quickly recycle and then move in to your new bigger tank. I would also run carbon as I somewhat recall sometimes toxins being trapped in a DSB but I cannot say for sure.
The reason I remember researching this was when I moved to Oklahoma, I too was moving a 4 inch DSB. Unfortunately, the movers did not have room for the tank which I requested to be put in last so I could restart. Thus my break from reefing. I hope that helps.
