How to change an existing Mass Effect 3 character's face

Create a new Mass Effect 3 character with the desired face (also see How to recreate your unimportable Mass Effect 2 character in Mass Effect 3).
Download the savegame editor Show archive.org snapshot (it's very beta).
Open the character with the desired face, go to "Player", then "Appearance", then click "Export head morph". Save the file to disk.
Open the character with the face you want to change, go to "Player", then "Appearance", then click "Import head morph". Load the file you saved before.
Save the changed character to a new savegame.
Resume the game.