Hi Jingua, using 404a v 502 is an assessment you need to make based on several factors, incl present condition of unit ; the type hoses, gaskets & seals it is built with & cost/kg of both refrigerant types + their respective oils / litre in your country.

Looking through your previous posts, I can see you have asked similar questions since Jan and again April 2006.

Is this the same unit having repeat issues? it's your father's ; it's your friend's.
Are you really a student or running a transport / service business?
Best if you just be direct with the forum ..... then we know better how to advise you.
If you don't really know what you are doing, my advice is pls consult some qualified HVACR workshop to overhaul it. Cheaper in long run than repeated " home improvement" attempts at it!
regards

regards