Unless your son has status he is not eligible to upgrade. So buying a new ticket in K will not help. I would work to try and get seats together for the inlaws and your son. Check out http://www.benedelman.org/travel/dlseats/ every day and look for seats together. Also, I believe you can find an agent that would assign preferred seats to your son and in-laws since you are on the same flgiht (some have reports less willingness for agents to do this) if you see preferred seats together

You can't combine PNRs

Happydad