What if tomorrow China changes his economy from oil-based to let's say Hydrogen or another alternative energy source ??

We have all the technology, but as long we do not apply it, we leave them an opportunity to out-race our already weak economy.

Let's start with private transport. I remember the story my grandfather told me about the lazy dog of the milkman in his neighborhood. One day somebody started feeding the animal pulling the small buggy and overnight others followed the example ... everybody started to pet and feed the dog.   Within a week the milkman and his dog zigzagged from door to door.
Grandpa always told me this story with a big smile.  As a kid for him it was funny but it was a real nightmare for the poor milkman who had to follow his dog, hunting for treats. He pulling the buggy to make his dog slow down trying to protect his milk bottles.

It is hard for me to imagine how it was in those days, I've seen some pictures of carts pulled by horses and dogs. This was transport a few years before World War I (1914-1918). In less then 100 years we moved from carts and bicycles to car and motorcycle. What a change in such a relatively short period. 
By the way it took less then 2 decades for oil prices  to rise from 10 to +100 $.

And still you're thinking the Chinese will not make this change from oil to something else?  I can't believe this.

Do you know when the first Japanese car was sold in the USA? 1956 (source : http://www.scaruffi.com/)  Back then, nobody in the USA would have believed me if I had said : "And there are many more to come!"
Every good American would have answered me : "Nobody can beat Detroit"
"What about 1973?"  First real oil crises.  Japanese cars used only half of the petrol compared to American cars.
Why Japanese cars are the best seller in USA ?  Because American car manufacturers were practicing what's called "planned obsolescence" and Japanese developers kept on improving and innovating.

The change is happening :
In 1997: Toyota introduces the Prius, the world's first commercially mass-produced hybrid automobile, (source : http://www.scaruffi.com)
2007: Toyota passes General Motors as the world's largest car manufacturer !!!

Japan is a very small country, it's population is estimated at around 127.4 million.  But what about China?
With just over 1.3 billion people (one out of every five people on the planet), China is the world's largest and most populous country.
And what about India?
India is currently the world's second most populous country with 1 billion people.  By 2040, India is expected to become the world's most populous country (by then it'll be home to over 1.5 billion while China will have a population of under 1.5 billion.) (source : http://geography.about.com)

They don't have the money to pay the energy-bill and in crisis people get creative. They will eventually find an alternative  for highly expensive oil consuming cars.
Honestly ?  What is more fun then driving your own car ? Anywhere in the world ...