The Chinese have said that if the US doesn't reduce its debt it will start offloading its US Treasuries/bonds - and when their export markets have expanded and domestic demand has grown they might well be in a position to do just that. Even the odd threat of it causes the dollar to plunge.
Quite frankly, the US debt market is exactly what allows Chinese to keep yuan undervalued by buying dollars.
As long as China wants to be an exporting country, they will have to amass dollars (or any currency of their trading partners), either directly (via CB) or indirectly (via outward private investment).
Economic adjustment doesn't happen overnight. By the time Chinese reorient their economy the dollar exchange rate would have already adjusted.
China is already in the situation where it doesn't need to export that much or close to the situation where it can dump the Dollar.