While I haven't done it myself, I'd start by exporting the decal from the library. You can delete the extra layer definitions and then import it back in. Comparing it with another decal with the proper number of layers will probably be insightful.
I usually just grab whatever Decal(s) I want to convert and import them into an empty PCB, then ASCII-out to the last version before Max Layers existed (3.5 - I think?).
Open in a new, clean PCB and export the now standard layer decals to a lib.