B757 CREW REPORTS POSSIBLY CUTTING THE CORNER AFTER RWY CHANGE ON THE SEAVU 1 INTO LAX.

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DURING THE SEAVU1 RWY 25L ARR WE WERE RECLRED TO THE SEAVU RWY 24R ARR. PRIOR TO THIS; WE WERE TOLD TO MAINTAIN 300 KIAS. AFTER RE-SELECTING THE NEW ARR IN THE BOX; I REALIZED THAT IT RESULTED IN 2 DISCONTINUITIES. THIS MADE IT EXTREMELY DIFFICULT TO FIX THE BOX AND FLY THE ARR AT THE SAME TIME. I COULDN'T TYPE FAST ENOUGH TO STAY UP WITH DOING THE ARR AT 300 KIAS! AS A RESULT; WE THINK; BUT AREN'T 100% SURE; THAT WE MIGHT HAVE CUT THE CORNER BTWN KONZL AND SEAVU. APCH SHOULD RECOGNIZE THAT THEY ARE REALLY PUTTING US BTWN A ROCK AND A HARD PLACE WITH THIS TYPE OF LAST MIN CHANGE!SUPPLEMENTAL INFO FROM ACN 736593: JUST PRIOR TO XING KONZL ON THE SEAVU RWY 25L; ATC SWITCHED US TO RWY 24R. THE FO HAD DIFFICULTY IN UPDATING THE FMS AND WE EVENTUALLY ENDED UP WITH A STRAIGHT LINE FROM KONZL TO SEAVU WHICH ELIMINATED 2 REQUIRED FIXES. IN THE MEANTIME; I DEAD RECKONED OFF THE INACCURATE FMS MAGENTA LINE APPROXIMATING THE POS OF THE MISSING TRTLE AND CATAW. AT SEAVU WE WERE BACK ON COURSE. THE LESSON I LEARNED IS THE UBIQUITOUS BACK-UP OF RAW DATA AND THE EXCELLENT TECHNIQUE OF HAVING THE OTHER RWY IN RTE #2 AS A READY BACKUP.