The reason I didn't believe it was because of ign's translation of Kazunori Yamauchi description of the game.
"Yamauchi provided a summary of what's known about GTHD so far, including the split in modes between classic and premium. Just to avoid confusion, we'll repeat the details here. Classic mode is GT4 in high-definition and a few other visual enhancements. The mode will be playable exclusively through downloadable cars and tracks. Premium mode is a single player preview of Gran Turismo 5, featuring a full-fledged PS3 graphics engine and detailed tracks and car models."
and since we all Know that they pretty much scrapped the second concept ( the one supposedly with the new models, damage and gt 5 engine again repeated here http://ps3.ign.com/articles/748/748610p1.html
) I am curious as to where you found out the information about the redone cars and engine. I am sorry I cannot read japanese so I did not come across it.
A lot change prior to the release of GT HD Concept.
It is a taster for GT5, and very nice too. If you want to get the most out of it, hook it up to a GT Force Pro. Unbeatable.