Real time digital audio frequency changing device
I think the best you could hope for is a vocoder: -



Input (modulation) is voice or guitar or generally any audio. The vocoder works by filtering the input into various frequency bands, converting those band-limited signals to amplitudes and using those amplitudes to control several voltage controlled amplifiers.

The input to each VCA comes from a band-limited filter that is fed from another analogue input source called the "carrier". The carrier is usually another musical instrument such as a keyboard synthesizer.

The output is the sum of all the band limited filters that have been individually modulated by band-limited versions of the modulation signal. Vocals can be made to sound robotic or very choral/chordal and incrementing up and down in semitones is child's play.

Used em and love em!

I'm sure there are projects in google-land that are available for this to be built.