I think you are confusing this with "RE-amping."
A re-amping device allows you to take a pre-recorded track (usually a naked DI track that was split off and recorded separately during the original guitar/bass performance) and run it back though a different signal chain at a later time if need be. The re-amping device changes the impedance and level of the recorded DI-track BACK to its original high-impedance, instrument level state, which the guitar amp/pedal/whatever is used to seeing from the guitar.
So, for example, if you recorded a guitar part with a crappy Peavey practice amp and later borrowed a Bogner, you could run DI of the original performance back into the Bogner, re-mic it, and record the original performance again with a much better guitar tone.
More details here:
Re-amp - Wikipedia, the free encyclopedia
Unfortunately, this sort of thing does NOT work with vocals. Although you COULD technically run a vocal part back through another signal chain, you'd simply be adding the characteristics of the new chain to the original recording, and you'd run a very good chance of degrading the sound quality of your recording.