How can I switch to an unlocked character without starting a new game?

We are playing Nine Parchments in co-op with the two starter characters, Gislan and Cornelius. We recently unlocked the first character variation of Cornelius.

How can either player now switch to the new character, without having to restart the game at stage 1? (And similarly for entirely new characters.)

A trick from here:

1. Have at least one other person in your game. Now you both have the game saved.
2. Leave the MP game and start a new single player game, pick your char, reset your skill points at the fountain if you want.
3. Delete your copy of the saved MP game by going to Resume, finding it, and deleting it.
4. Have the other player resume the MP game.
5. Join their game. You will now get to choose your char and spells.

Although normally, you have to start a new game to use unlocked characters.