Are there any circumstances under which a Retired service member in Category VI may be upgraded to a higher category?

You bet there are. If you are traveling Space-A overseas and an emergency occurs at home, you may be upgraded to Category I, Emergency Leave, Unfunded Travel, by the installation commander or his representative under para 7-C, Chapter 6, DoDD 4515.13. However, you should have the emergency verified, in writing, by the American Red Cross before attempting to obtain an upgrade.