1). Is there a connection to SFO for the earlier flight? If so then rebook the whole flight, then you have a backup if that flight is late and connection is missed.
2). Try to go standby on the earlier flight out of BOS. If you call before you head to the airport you can get an idea of how possible that is. The CSRs are not supposed to tell you how many seats are available (more or less than 9 is the official answer) but will often be more helpful, if you talk to them.