I wouldn't say this change will occur any time soon, but I believe inevitable. US is the worlds largest economy partly because it has the population to sustain it. Simple math is simple math, China has over 3-4 times more people US has, if they are able to catch up in terms of a middle class, GDP/person, US just can not compete. And not even the best policies will help. China just needs to ensure it's economic progress is not a bubble and slow it down to a more manageable rate. It has a few more crucial tests, a currency that's not fixed for one.

And you can bet the Chinese are going to match dollar for dollar what the Americans are spending in military, eventually surpassing them.