I suspect that the 5-minute change was in anticipation of or part of the aircraft sub. By the time OP called, there were no seats. So, that means a reroute. No idea whether a later BOS-SFO works for OP's schedule because his onward flight to Area 51 (have to presume that is "xxx") would have to move as well, but if it's available UA will certainly do that.
But, UA certainly may shift the routing. OP's contract is BOS-XXX and that is what he still has. But, he may cancel for a refund and that is what he says he will do.