What are the fare rules/advance purchase requirements for the B fare?

If the OP is flexible, and has something like an expertflyer subscription, the assumed refundable/changeable B fare might be worth considering. I cleared west coast to SYD by switching from LAX-SYD to SFO-SYD after getting a R=1 notification five days before the travel date (and I routinely see R>0 around 12-23 hours before the flight, unfortunately often at times when I am sleeping)...