If I buy a ford truck, and jack it up, put a light bar on it, custom paint it, tweek the motor and do all sorts of wacky stuff Ford doesn't sue me for violating their "IP". They don't ask me to take the name off of it. In fact, if I make kits to modify the vehicles they are happy as a clam, as it sells more cars.
If you make a kit and sell it as Above's Kit for Ford Trucks, that's OK. If you sell it as Ford Truck Kit, Ford would most likely claim a trademark violation. This is the LegOS distinction.
Also, if you published how you tweaked your engine and GM used that info to improve Chevy trucks, there may be a problem.
Also, Lego seems to be worried that adding that light bar may make the truck unsafe and reflect badly upon Ford.
If I buy a house, add an addition, change the colors on the walls, swap out the heater, and then tell people it's a "Toll Bothers Hose" they don't sue me for violating their IP.
Because you haven't violated their IP. If you took the floor plan of your house, modified it and posted it on the web as a Toll Brothers' floor plan, they'd have room for trademark infringement.
And if Above Builders, Inc. used that new floor plan to build homes, there could also be a problem.
Again, if your house comes crashing down because of your modifications, don't blame Toll Brothers.
