I dont think you can. You might be able to do a mesh swap though.
Revert to additives, Select all for the island → create static mesh. Create the replacement sm, right click the block version → re-import, select new mesh.
You should use procedural foliage volumes, manual adjust/edit. Volumes don’t care what’s under them… for the most part.
Otherwise you’d wait to do foliage until the permanent mesh is in place.