What will happen when bupropion xl is mixed with prescription pain killers?

Not much interaction: if taken in appropriate doses and prescribed by someone who knows your whole health history. In higher doses, bupropion has a risk of seizures (usually in doses of 450 mg a day or more), but some people with predisposition may get seizures at usual doses. Opioids may add to this risk.

Generally, you're ok: Your doctor should be aware of your current meds and adjust dosing of all of them to deal with acute pain. Opiate pain killers probably aren't helpful in the long run for chronic pain.