Paid F fare ans just for this reposnse from the twitter team trying to SDC to an earlier BOS-ATL flight (get ahead of possible weather later). Maybe it is the J fare throwing things off but have done it before when there is only I and J available. Also she claimed she asked a supervisor and supervisor said it was impossible. Only standby available:

My apologies for the delay in response. I can only list you on standby for the earlier flight, because your class of service is not available. You booked an "A" fare, and I only have J available on the first segment, and F on the second segment. Would you like for me to proceed?