It will depend how quickly your status updates. They will be available as soon as it does but if you are, for instance, relying on double TPs from a BAH, then almost certainly you won't be elevated to GGL in time to use the pre April 25 Jokers. But theoretically, yes, if you make bookings using the Jokers then you could change them in the future, but the bookings would need to be for the same passengers.