What is the ruling on a woman piercing her body, including her 'awrah, and what is the ruling on the piercings she made before embracing Islam?

There is no objection to a woman using pre-existing holes in her body to hang adornments, provided she does not display them to non-mahram men, and that it does not involve imitation of disbelievers or immoral people. If the adornment is in places that are famously associated with immoral people, then it is not permissible to hang it.

As for the ruling on undergoing piercing operations, it is forbidden if it results in the exposure of awrah, or causes health harms, or involves imitation of disbelievers and immoral people. It is absolutely forbidden for a man to undergo piercing for the purpose of hanging adornments.

If piercing operations are free from the aforementioned prohibitions, they are permissible if adorning in that specific location is a common custom among women.