Infant Child Seat Harness Buckle problems of the 1998 Ford Windstar

I purchased the evenflo medallion carseat (model #2541113p1, manufactured 25feb00). My 16 month old son is able to unclasp the chest buckle. I have to to stop every few blocks to relatch it. I contacted evenflo about my problem and was told that the buckle had been changed on the newer model and they would send me the new buckle. But, if I use this buckle and it does not hold, evenflo would assume no liability since I had altered the harness. They said this buckle had not been tested on my model carseat so they would not guarantee its safety ( even though they offered to give it to me, interesting don't you think?). I was told by their customer service rep that the best thing to do would be to buy a new seat. This seat cost about $100, and I purchased it because I was told that a five point harness was the safest. I truly cannot afford another seat ( although I will buy one if it is my only recourse). Please advise me on my next step. Thank you for your time.