hmm I have thought about something What would happen if you are inside him while he is a car and he transforms himself to his dragon form?

That's a good question. If he were a Type C cargon which the vehicle size would be larger than the dragon's body, transforming down to dragon form will result in his occupants and cargo being absorbed as temporary fat distributed along the belly and flanks and when the cargon turns back to vehicle form again, you'll be back in your same seat unharmed. The cargon can also spit you out if he/she wishes.

But if you're referring to Jeff going from Cargon back to Latios form, then you would just fall out as he's transforming back to his natural form. This is because Jeff's cargon form is a creature card can can only be accessed when he has the card equipped in his suit.

hmm this is actually fascinating and something that I would like to try this out sometime

I need to get around to designing a cargon character. I love all of them XD

If you're wanting to create your own oc of my species, you'll have to talk it over with me through notes.
Glad you love em'

Oh I'm sorry. I didn't realize they were a closed species.
I would not mind noting you about it if you would be okay with that tho I would need some time to formulate a solid idea (unless that is what the noting is for)