OK, you said they're bottle-conditioned, so yes, try to warm them up and see if they yeast will clean up after themselves. That's probably your best case scenario.
Hard to jump to the conclusion that it's infected. Pedio is what typically makes diacetyl. If you had an infection, you might see addition confiming signs: haze, higher carbonation, sourness, etc. If that happened, you're pretty well hosed.
How is the diacetyl presenting itself? Aroma, flavor, etc? What does it taste like? (In other words, are you sure it's diacetyl and not caramel?)