Make new ones and swap them in. Totally possible.
Not likely though, but I've yet to see what PD does with support for the entire life of a game. This is the first one that's been patch-able (minus GT5P which I didn't get)
I really don't think there should be a DLC for this at all. They would only be making what's in the game better (or, in a certain light, bringing it up to what should be standard). Therefore, it should be either a free patch (or free DLC as this would likely be a few Gigs and I wouldn't force anyone to have to download that much if they didn't want to) or they should just forget it and work on modeling cars that aren't already in the game.
But if they did charge for it, I'd probably shell out $20 tops. I don't feel like I should have to pay for upgrading what I just payed for. I'd rather pay for an addition to it or have them upgrade as a courtesy. But I do love GT5 so I'd probably cave and buy it (boycotts never work anyway).