Thanks for the question. Yes. If it's the same type of device (of the two types, iPad or iPhone), you will not be charged again. You will be unlocked for free. Just follow the normal purchase process.
To be clear, if you were unlocked already on iPad, you will not be charged again to unlock on iPad from the same App Store account.
If you were unlocked already on iPhone/iPod touch, you will not be charged again to unlock from iPhone or iPod touch.