antigramadj.
An anagram in which the new word or phrase is the opposite of the original.
Example Citation:
"The converse of the aptagram is the antigram. In antigrams, a word or phrase gets rejuggled into another word or phrase that bears a meaning opposite to that of the base. Thus, astronomers/moon starers is an aptagram while astonomers/no more stars is an antigram."
Richard Lederer, "Looking at language; Check out this ever-changing parade of antigrams," The Patriot-Ledger, April 12, 1997