I request to know how I can get a free space available, or low cost upgrade from Economy to Prestige Class based on medical need.

I will be ** years old on * April. I have heart disease and must wear a pacemaker. If I must sit for an extended time, my legs and ankles swell and cause pain even with compression socks. If I can recline, there is no problem. I'd like to fly from Thailand to Atlanta, Ga. for my birthday there in April. I can fly on any day of the week. If I buy an economy ticket and there are seats available (often the case) at departure time in Prestige or Business Class, can I get an upgrade for medical need, either free or very low additional cost? I made this flight last year, Prestige Class, using my Korean Air Skypass: ba******** member ID, JonEdwards air miles, and it was wonderful. I'm on the family plan, and I think they used all my family miles, but I'm not sure and don't know how to find out. I don't have enough money to pay the full Prestige Class fare. I request to know how I can get a free space available, or low cost upgrade from Economy to Prestige Class based on medical need.**Thank you,*Jon Edwards

Many years a customer.