Yeah, you can't change a vehicle's datablock - well, you can, but it often causes people to crash.
What you need to do is create a script to register a new datablock (derived from the jeep) with the shape file of a Porsche 911. I don't personally have a shape file for a Porsche 911, but you might.
If you need help look in this forum (specifically at the topics about creating datablocks) and the Vehicle_Jeep server.cs script. It'll just open with notepad, but a program like notepad++ or sublime text 2/3 would probably make things easier for you.
For making a Porsche 911 shape file from scratch, you might want to use a program like Milkshape or Blender. I'd recommend Milkshape as the simpler of the two, but Blender tends to be more reliable these days.