That's pretty much how it goes. There's only one chance to get it, so if you get rid of it, you're just out.
Personally I think after you win a prize car it should be either added to the Car Gallery (or whatever it's called), or just made available for purchase. They all have 'sell' prices, and we know that in GT3 cars sell for 1/4 what you pay for them. Easy math seems like to me.
I say this because as I'm putting miles on my 787B, I'm thinking about getting a replacement ... but there are only two places to win them (that I know of), and neither can be repeated.
YIKES!
~LoudMusic