transition has 4 distinct synonyms spread across 3 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 act of passing from one state or place to the next
a change from one place or state or subject or stage to another
cause to convert or undergo a transition
“the company had to transition the old practices to modern technology”
make or undergo a transition (from one state or system to another)
“The airline transitioned to more fuel-efficient jets”
Synonyms, antonyms and the broader and narrower relations for transition come from Princeton University’s WordNet. Full sources and licences.