Likely cost of upgrading "K" PE to "S" or "W" PE?

Tried calling the FC this afternoon and couldn't get through, so thought I'd ask here;
I've got a PE ticket to LAX dep 3/6/11 returning 8/6/11 that's a K fare. Anyone got any idea at all what the likely range (as I know this is NOT an exact science!) of the cost of upgrading the K to an S (or W - but I assume the S would be cheaper) might be as I've got a ton of miles sitting in my account?
Thanks.
I've got a PE ticket to LAX dep 3/6/11 returning 8/6/11 that's a K fare. Anyone got any idea at all what the likely range (as I know this is NOT an exact science!) of the cost of upgrading the K to an S (or W - but I assume the S would be cheaper) might be as I've got a ton of miles sitting in my account?
Thanks.