We'll need more information.

What sort of credit voucher did you use for the original purchase? Was it $$ off or a free ticket?

What we can say:

Failure to fly the first leg will cancel the entire itinerary.

Your best option may be to rebook the ticket as a Illinois-SNA ticket, then buy a LGA-Illinois ticket that 'completes' the travel and you fly back SNA-Illinois-LGA. You're probably looking at a $100-$200 change fee to do this, plus fare difference. Trying to keep one leg of the travel the same is probably less of a benefit than the increased cost of an open jaw ticket.