From a good source regarding LADWP (who shares power to SCE) upgrade going on at Sylmar (and Celilo) for the DC Intertie...
"The project consists of removing the existing two AC and two DC Filter Banks at Sylmar Converter Station West; replacing the existing two AC Filter Banks with four new AC Filter Banks and two new DC Filter Banks; upgrading the capacity from 3,100 megawatts (MW) to 3,224 MW and upgrading the existing HVDC Control and Protection System at Sylmar Converter Station East."
So even though San Onofre didn't really produce power for the same footprint of what LADWP and SCE serves, I don't think Metrolink (or anyone else for that matter) will see any issues.