PD would have to reacquire licences for every car in the game...
This...
But for the sake of staying in topic, let's say that licences was not an issue.
The first two GT would never be remade. Most likely all we could ever get is a PSOne classic on the PS Store. I'd be happy with that, so I could take it away on my PSP.
GT3 was the start of something new, but that wasn't the full package of what Gran Turismo could really do. Even though is the most popular, if both GT3 and GT4 are placed in a shelf, GT4 is usually the one to be picked due to being more recent and more popular with the younger generation of gamers.
So, GT4 would be the one to be remade into HD...but wait! Isn't this game
already in HD? If I remember correctly, this game was lunched later on in the U.S. region with an option to run at 1080i. With that said, I don't think it would be necessary to make it again to the rest of the world. Weird, I know, but that's how game business goes...
As for the Concept and GT4 Prologue versions...those will never be in the light of HD, let alone launching them again.

It would be pointless, since Concept was a special game to showcase the recent cars being showed in the 3 major auto events (Detroit, Geneva and Tokyo), it was lunched
exclusively in European and Asian countries.
And the GT4 Prologue would even
more pointless since that GT4 in progress. So why launch a half finish/demo product again?
The only game from the series to have a pontencial remake or comeback would be GT PSP. Possibly to add new features for the Vita or upgrade some other relevant stuff...but that's less likely, still, more possible than the rest of the games.
Now...who read all that jibba jabba that I just said?
