• player is a Player object
• oldCharacter is an object (specifically bot.players[player.id].character)
• newCharacter is an object (specifically bot.players[player.id].character)

bot.on('playerChangeCharacter', (player, oldCharacter, newCharacter) => {
    console.log(`${player.name} has changed their skins`);
    console.log(`old skins:`, oldCharacter);
    console.log(`new skins:`, newCharacter);
});