It depends if you want to transpose the whole thing or just switch modes to C. Very simple with Melodyne either way and if you know how to use it properly it won't sound too auto-tuney. Patience helps.
The way I do it is to just use the second pitch nob in the channel settings for the vocal. Set it to "Auto" rather than "Resample" or it'l stretch. You can get a good range of different pitches this way without stretching the vocal. Sometimes it can sound odd, but most the time it should sound alright.
As for changing the vocal sample directly on the piano roll, I'm not sure it can be done. You can try using the Newtone plugin, but I always found that it screwed up the sound.