While poking around for some free MMO's I ran across one called Red Stone. Each class has a transformation ability that allows them to switch to an alternative class with enhanced abilities under certain circumstances. One class, the Princess, can transform into the Little Witch. According to the class description on the official site:
Quote:
A Princess is able to transform into a Little Witch, going from a small girl to a full-grown adult instantly. Little Witches use strange and unique magic based on light and sound. This lends to the rumors of their interplanetary origins, as the stories say that their magic comes from the power of the stars themselves.
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Basically, it's your classic "magical girl" type thing.

The Princess/Little Witch class page can be found
here. It's doubtful there's any actual process (I'm still downloading the game so I haven't played it yet) but I thought it worth mentioning here anyway. Especially since many of the classes have transformation abilities such as the mage being able to turn into a werewolf, the priest into a fallen angel and the necromancer into a demon.
Besides that, it's not a bad looking game plus it's free and has very low system requirements.