It depends on how it's ticketed. Could be a broken fare, could be a stopover. If the former, just the final segment would be repriced, if the latter, the whole ticket will be repriced (in which case it might be easier/cheaper just to buy a new ticket SFO-LAS).

Or you could see if you're able to SDC SFO-LAX to SFO-LAS-LAX and then just walk-away from the final segment :-)