Any tips on dissolving/removing a subsidiary in Netsuite that has both an active child subsidiary and active parent subsidiary? how do you restructure the subsidiary hierarchy so the child subsidiary is now under the grand-parent instead of the current parent it's been under for the last few years

There's a hierarchy change tool, but note that if the new and old parent have differing currencies and/or if there are eliminations involved, it could create headaches. Definitely test in Sandbox