When Downgrading a Property, move backwards along the appropiate progression list from your starting Property. Moving once per Downgrade.
Example: The Exploding Property being Downgraded would become the Fine Property. The affected Pool would no longer be under the effects of the Exploding Property and now only be under the effects of the Fine Property.
The same would be true in the case of Disadvantage being Downgraded to Fickle.