It may depend on the special offer itself. If you are not past the PIF date, you most likely can apply a special military offer to your current reservation, unless it indicates "new reservations only." If you are past the PIF date, I've read of some who could have a special deal applied, but other times no. Some have found that a special offer puts $$ back in their pocket even with accepting the penalty for a cancellation after PIF so you would want to do your math and see how it works for you. I think some of the past military offers were only for specific dates/itineraries so while an offer may come out it may not work for you.