IIRC if you modify the retention category it is retroactive, but if you change the policy to be applied it you would need to associate the new policy to the historically archived content. This thread has a somewhat confusing explanation of it but my recollection I think is basically the jist. Special consideration for storage devices where EV then sets the retention on them.
I just looked at the PS commands (which I am mostly unfamiliar with but have a recent interest in) and do not see anything to help you. that said I know there are third party products that can change historically archived items retention and I believe you can do this in teh DB (after a backup with bravery and out the guidence and assistance of your friendly support team if it goes belly up)
