The thing is what will America do?
Russia is a country with UK like economy. If China today wanted to seize all UK international assets within reasonable reach, it could do so. It would cause an uproar and many countries wouldn't approve, but it'd be no different than what happened between US and Russia.
China on the other hand is the world's largest economy. Dollars have value largely because China is ok with dealing using them.
Much of China's dollars are stored in itself or allied countries. Even if some stooge in the white house says these dollars are invalid, these dollars still exist and can be used.
Another is the issue of debt. US has massive amounts of assets in China. If US defaults using war as an excuse, China can just seize their assets. If China needs to extract a pound of flesh from America to pay back the debt, the stuff is already there, inside the mainland, on HK, Taiwan, wherever, ripe for the taking.
The paradox of reserve currency is that the current arrangement lets America borrow China's geopolitical power. But at the same time, China needs to develop the export goods market to improve the nation.
I'm no economist but the way I see it, the ideal situation for China is to avoid contesting dollar reserve currency but get a yuan resource monopoly. Then when Chinese brands comprehensively dominate most of the global market and only after US has been guided to a soft landing like how Gorby landed the USSR, then discussions about a shared Yuan and dollar global reserve will be made, after Chinese companies picked up the scraps of what's left from US former national champions using devalued dollars.