Will once again re-up the concept of a “right to root access”, to prevent big corps from pulling this bs over and over again: https://medhir.com/blog/right-to-root-access
So it's actually far from trivial to draw a line.
Cargo van -> Camper van conversions go through this all the time - you add/remove seats, add a lot of weight in the form of beds, water tanks, etc. add/remove windows, put solar panels on the roof... After those changes you have to take it down to the vehicle inspection, and they tell you whether or not your changes have been deemed acceptable to drive on public roads.