WordNet records no synonym and no antonym for migration. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the movement of persons from one country or locality to another
a group of people migrating together (especially in some given time period)
Synonyms, antonyms and the broader and narrower relations for migration come from Princeton University’s WordNet. Full sources and licences.
(chemistry) the nonrandom movement of an atom or radical from one place to another within a molecule
the periodic passage of groups of animals (especially birds or fishes) from one region to another for feeding or breeding