B757 FLT CREW HAS A CLOSE-IN RWY CHANGE DURING CIVET 5 ARR TO LAX.

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ON THE CIVET ARR TO LAX WITH RWY 25R LOADED IN THE FMS. PASSING THROUGH FL180 WE WERE SWITCHED TO SOCAL APCH AND ASSIGNED TO DESCEND VIA THE CIVET ARR TO RWY 24R; CLRED DIRECT RUSTT INTXN. I PREVIOUSLY LOADED THE CIVET ARR; RWY 24R IN RTE 2 AND THEREFORE; WITH THE CAPT'S PERMISSION; ACTIVATED AND EXECUTED RTE 2 AND DID A DIRECT INTC TO RUSTT. UNFORTUNATELY; ALL OUR ALT RESTRICTION (PREVIOUSLY CHKED) DROPPED OUT OF THE FMS. FROM THIS POINT WE SET THE ALTITUDES MANUALLY/USED FLT LEVEL CHANGE AND FLEW THE LATERAL TRACK OF THE FMS IN LNAV. AT THE SAME TIME OF THIS EVENT; SOCAL GAVE US TFC TO MAINTAIN VISUAL SEPARATION ON AND SEVERAL SPD CHANGES WHICH MADE SOME OF THE ALT RESTRICTIONS VERY DIFFICULT TO MAKE. ALTHOUGH THE CAPT AND I FELT LIKE WE WERE VERY WELL PREPARED FOR RWY CHANGES; WE BECAME TASK SATURATED UPON CONTACT WITH SOCAL DUE TO ALT RESTRICTIONS DROPPING FROM THE FMS AND DIFFICULT SPD RESTRICTIONS GIVEN WHILE ATTEMPTING TO MAKE ALT RESTRICTIONS. YES; WE COULD HAVE ASKED FOR RELIEF; HOWEVER; THE FREQ WAS PRETTY CONGESTED AND SOCAL SEEM TO BE TASK SATURATED ALSO. WHILE I HAVE THE UTMOST RESPECT FOR SOCAL AND THE VOLUME OF TFC THEY HANDLE; THERE HAS TO BE A BETTER WAY OF GIVING THE CREWS A HEADS-UP OF WHAT RWY THEY CAN EXPECT AND AVOID RWY CHANGES BELOW FL180 IF POSSIBLE.