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That's good news that the algorithm is decent. I was talking about shifting at least a whole note, a total key change in back.

What is an artifact, by the way? Sounds not so good. Anyway around it?



Artifacts = un-natural sounds introduced from overdoing it.

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Do the programs such as TC Helicon or Autotune allow key shifts as well as pitch correction?



In a nutshell, yes they can be used for that.
TCHelicon can be used by using a midi track of the notes you want sung.
Autotuners can be used to force notes to other notes... by selecting scales, etc

Going a half step is usually no problem, going a whole step is about the limit if hoping for no artifacts.. usually. Sometimes you get lucky/unlucky in either direction. Experiment and have fun.
Easiest way would be to make a wave file of the song, then tune it down a step by right-clicking the wave display in the tracks view and selecting time shift/pitch shift, sing your part (without worrying about if the mix sounds OK tuned down), then export your vocals to the project and pitch-shift up to see if it fits.

I do not work here, but the benefits are still awesome
Make your sound your own!