Revitalizing an Old Reef

[DeeFromBrooklyn]

My 125 gallon system is over 15 years Old. At one point it was over run with soft coals but now I have removed 70% of them and kept a few leathers which aren't doing well at the moment and several mushrooms. I plan on going to a larger sump, auto doser and calcium Reactor. I run a 250 gallon rated SC Skimmer. The fish have been with me all these years. I'm not sure whether I'm replacing or removing all the old sand so I started removing a little at a time. I don't think I need to keep running the refugium since its pretty small area in my sump
Has anyone else done a tank overhaul? I've attached pictures of it full now a bit empty. Time to change but I fear the cycle Can anyone share since creative pictures

 

[Lazydayz]

I have started taking the sand out of mine as I want to add more SPS and like the flow. I've been doing it extremely slowly since BRS said that they crashed the 160 trying to go bare bottom. I'm taking about a 1/4" of sand out in small spots once a month. Seems to be doing well no negative effects that I can see.