After the expiration date of the credit, you can call customer relations and they will issue a LUV voucher for total - 75 fee and anyone can use it. I believe the expiration of the voucher will be 6 months. Of course, once you but the a ticket for a new person, the funds then get locked into that person's name.
Also, any ticket which has any portion of the ticket paid for with the LUV voucher cannot be further extended.