I was wondering if car packs, like Cj24s that are made for ELS v7 will work will ELS v8

They will. There is really no difference between cars that were made while V7 was out and cars that were made after V8 came out.

That's not quite true. Coronas in ELS 8 are placed in the position of various dummies on cars; for most, they should already be in the right spot (because IV uses them too), but for extras 5 and 6, coronas may be in the wrong spot.

However, you can always not use coronas, at which point this isn't an issue. You'll just have to update the ELS vehicle configs.