When having bought a new head, keeping the body - or having bought a new body, but want to keep on using one's head, it is often recommended to use the shape asset that comes with the parts.
It would simplify things if we could merge shape assets, as in, selecting two shape assets, choosing "create merged copy", and then select categories and / or values from the one or the other to keep in the new copy.
This should of course respect the permissions from the most restrictive asset chosen: no-mod, as well as no-copy shape assets cannot be merged at all. If any of the shape assets have no-trans, the resulting shape asset will also be no-trans.
Possibly related posts, yet not quite the same thing: