B737-700 flight crew missed their ATC assigned crossing restriction on arrival to LAX.

Narrative

On the arrival; we were set up for 24R and were told to cross RIIVR at 13;000 and expect 25L. We requested 24R and was told it didn't look good. The Captain started to set the FMC up for 25L and; approximately ninety seconds later; we were told to cross SEEVU at 250 knots and then cleared for the 25L approach. We were at 280 knots previously assigned. I then went heads-down to finish setting up for the 25L approach. A few seconds later; ATC asked us to start our descent to 13;000; which is when I saw we were high on the arrival. The Captain started the descent with the speed brakes. We would have met the crossing; but then we were asked if we were at 250 knots. I confirmed we were at 280 and told to slow to 250 at SEEVU. We were then told he wanted us at 230 knots immediately. We started to slow and I realized we would never meet the crossing. I informed ATC who then gave us a heading south of the arrival and we were told to descend to 11;000 and to expect a turn back on course in one minute. We were then turned back in and landed without incident. (Of note; the Captain later told me we were given relief for the crossing restriction to slow; but I never heard it.) We never missed a crossing because of the vector after informing ATC we would miss it; but it was enough of a cluster to file an ASAP Report.