1st option- call SCE and speak with the planner for that area and ask if they will take care of it for you. They should already be coming out to power you down and back up. If I'm understanding your situation correctly, the wires that will be to short are the laterals, hence owned by SCE.
2nd option- using split bolts temporarily extend the wires within the panel to a length necessary to reach until SCE performs the upgrade. Taping correctly of course. Follow-up on this, I've revisited jobs a year later and somehow SCE lost the work order.
3rd option- This is the option I would use, considering other factors. Such as, is this a recess mtd all-in-one? If relocated would the stucco have to be repaired. Obviously I know your in CA by those 2 questions. 99% chance the laterals are direct burial to a stub from the bottom of your panel. If circumstances permit, install your new panel next to the old one with a new stub and temp power the new panel from the old until SCE gets out to trench in your upgrade. Talk to your SCE planner though, because often when you upgrade they don't. Kinda seems pointless?
If you temp in, that wiring will have to be to code of course.
I'm sure others will have more ideas..
