Russian has different verbs with different prefixes.

Изменять / изменить means "to change" something when a thing stays the same, but its quality changes.
Заменять / заменить means to replace a thing with another one.

Therefore, when applied to a word, "изменить слово" is normally understood as "to inflect a word", i.e. to change its grammar form (the word is still the same). If you want to select another word which is less archaic etc., you would say "заменить слово (другим словом)".