The unloading has to be done over some time.

{and if China use the dollars to purchase stuffs from around the world, like oil etc, that will perpetuate the dollar dominance. }

Use the dollar to buy oil and gas from murrika, while reduce China's own oil production-- i.e. saving China's own oil underground. Also, use the dollar to buy iron ore from Australia-- don't think they will accept RMB anyway. Etc. etc.