I have done many upgrades in residental and commercial, but this is a resi question. In a panel that is fed underground, how do you get enough of the feed wire for the new panel. I know the utility company needs to upgrade too, but that never happens the same day that I am there.All of the updrades I have done were all overhead feeds. thanks, Joey

Time to dig out enough of the earth to get our cover off for a little exercise in being very careful. Hopefully we don't have to get out the rotary-hammer for that patio!
You have to be quite careful not to make any slips. 100A to 200A we have to enlarge our hole a bit, and it would be nice if the penetration is in the same location to suit our new purpose. Now, we got our tin-snips, hammer and big screw stick, and the sawzall with the nice long blade. First we have metal to open, then brick/wood to stop the effort. Mind you being careful as a skilled electrician always is, nothing to stop the ground fault but a transformer.
Definitely not a fun task, I always dread when I find out the upgrade is an UG.
And being bit here and there doing these is always a high percentage. Been in the middle of it when the NorEastern blew in, bad gutters letting the overflow cascade on you kneeling in the mud puddle you helped to create, slicker-gets in the way, rubber gloves-can't get a grip on anything, gonna get hit-probably, just make sure you ain't gonna get sucked in for the long JOLT!!!

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..