Noah, So you mean removing the UGF and then changing substrate? You mentioned a layer of sand over the substrate. I assume you never vacuum substrate? Why do cories need sand?
My plan for now is to do weekly, 30% water changes, add more crushed coral, maybe a limestone rock and wait. If nothing changes in a couple months, I'll probably try adding Seachems Equilibrium. If that doesn't work out, I will either live with the set up I have or remove the UGF and change the substrate. After 20 years a new look would be nice. Wouldn't mind adding some rooted plants too but I don't want to get to the point that have to add c02. I'm a low maintenance guy.