Personally, I don't think I should have to pay for the conversion of Standard cars to Premium status.
Yes, I totally get that such would be a vast and expensive undertaking by PD to do.
I'll gladly pay for additional content for GT5 - big things like new cars and tracks, and even little things like new PhotoMode locations and driver uniforms.
However, converting Standard cars to Premium is not additional content as I see it. Truthfully, they should've made more of the Standard cars as Premium to begin with. (Only 223 out of the 1,034 total cars - or 21.8% - are Premium? C'mon!)
Some of the Standard cars are cool, and some of the Standard cars are needed to advance in the game. Beyond that, they're pretty much "filler" and were probably added simply to tout that magical "over 1,000 cars" achievement.
(Besides, I only have like 90-ish% of the Standard cars I have because I'm trying to collect all the cars. I don't wanna pay for cars already in the game that I can't be bothered with.)
Peace...