A synonym for the word "octavate" is "transpose". This means to change the key of a musical composition by moving all the notes up or down a certain number of notes on the musical scale, often eight. Another similar term is "transcribe", which means to write down or notate a piece of music in a different key. "Rearrange" is another synonym, which can refer to changing the order of the notes within a musical composition or rearranging the composition itself in a different key. "Modulate" is another related term, which refers to smoothly transitioning from one key to another within a piece of music.