Air carrier inbound to LAX described an excursion from Class B because of an ATC assigned altitude.

Narrative

Planned; reviewed; confirmed and briefed the RIIVR 2 and ILS 25L arrival and approach. SOCAL cleared us for the RIIVR 2 and ILS 24R. I think the controller heard hesitation on the read back and she offered that when we got closer in they may be able to change us to the ILS 25L. Captain responded that we just had to re-enter and re-check it all. Closer in the controller had a conflict developing and he canceled our approach clearance for ILS 24R and gave us a heading and clearance for the ILS 25L. He gave us a clearance from 7000 to 2500 and asked us to expedite through 4000. He noted he was trying to get us down and would have a plane over the top of us. Controller noted that the speed assignment and low altitude would take us below the BRAVO airspace and then we would be back in the Bravo airspace. I know that LAX used to pursue certificate action for inadvertent deviations below the Bravo airspace. Since we were under SOCAL control vectors and assigned speeds I assume this is not an issue.