Yes, I can see this is annoying, when there appears to be seats going on other services. I must admit I would probably book my own seat to LAX and ping BA for it later, if confronted with this response, but that's easy for me to say. If the flight had been cancelled then the agent could have made the full booking. It looks to me that you were auto-confirmed on to this service, which in some respects is good, but in other respects reduces your options. Furthermore you are only supposed to be allowed one involuntary reroute, you can't keep hunting for options, which AA is entirely happy for you to do.

The way this works must look very complex to outsiders, but what it boils down to is that yes the agents at SFO can do a lot more for you, including re-routing on to the LAX service. Telephone agents are heavily constrained as to what they can do, particularly if the aircraft is not showing to them as Cancelled (JB only, limited rerouting). But what you also have to understand is that late at night, with a full aircraft load of passengers, they really don't want to rebook hundreds of people on the night unless there really is no alternative. They probably would rebook First passengers and those with high CIVs or the corporate High Values.

If you get to the airport at a reasonable time today, they may be prepared to rebook you on to the 747s, but I suspect they may still be reluctant.

When you claim your 600€, you could of course point out that had you been rebooked on the night you would probably not have bothered to claim compensation had the airline been more accommodating.