replacement has 12 distinct synonyms spread across 6 of its 6 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 furnishing an equivalent person or thing in the place of another
“replacing the star will not be easy”
an event in which one thing is substituted for another
“the replacement of lost blood by a transfusion of donor blood”
a person or thing that takes or can take the place of another
filling again by supplying what has been used up
Synonyms, antonyms and the broader and narrower relations for replacement come from Princeton University’s WordNet. Full sources and licences.