From a recent CBP website posting:
The Generalized System of Preferences (“GSP”) trade preference program is due to expire on July 31, 2013. CBP has no information as to whether or not this preference program will be renewed and if it is renewed, whether there will be a lapse period, or whether there will be a retroactive clause providing for a refund of claims made during such a period.
When this last happened in 2011, Congress ultimately acted to retroactively extend the GSP and allowed for reimbursement for duties paid on GSP-eligible merchandise that was entered during the expiration period. However, Congress simultaneously increased the MPF rate to offset the estimated cost of lost tariffs associated with expanding the GSP benefits.