If you search various forums you will see that, in general, you always lose when you convert one airline's miles to another. If you didn't lose, FF programs would have no loyalty because you could simply fly whatever airline you wanted and then convert the miles.

You can convert miles through, say, Milepoint.com or Hilton, but you'll always lose a lot of value.

The better advice is to top off orphan accounts with credit cards or transfers (such as ClickMiles) and then redeem an award. You'll get a lot more cents/mile than if you flush an account through a conversion.