[I will give you some info from personal point of view. Last year in July, I took apart a fully stocked 120 full of sps and other corals and transferred them to a 215 with 5-6 inch new sandbed. I used the same liverock, same refugium,(undisturbed), I seeded the new sandbed with sand from the old system. I had problems for almost a year with, cyano, diatoms, and some turf algaes.
This year in July because of the previous tank leaking I had to redo the whole system again. This time however I did not use any sandbed, instead going with the so-called bare bottom method, using the same amount of liverock, same refugium(still undisturbed), same equipment. I siphon out the detrius as I do water changes. I have not seen any sign of above mentioned nuisance algaes that were so prevalent last year after the change. I do have algae that grows on the glass wih abundance, but I guess you always get that,(the 2200 watts of lights aids that growth).
I say this, because I believe if you take a fully stocked system after a few years, remove the sandbed, put in a new sandbed and continue on. The new sand bed will take a year or longer to catch up to the system. It is not like taking a new sandbed and slowly building your bacteria and pod load as your bio load of fish and corals(thus feeding increases). You are instantly adding a full bio load to sand bed that can not handle it.]