Changing Retention Category for archived messages

The majority of our users are in a single retention category (Retention still indefinite).  Management has finally decided to provide a timeframe for expiry.  Prior to this expiry we need to now move some users from our standard retention category to a category which will have a different expiry.

Is there a way to change the stamped retention category of mail which has already been archived without exporting the archive, delete the archive and then re-ingest with the new retention category?

Thanks

Paul