Why are Chinese exporters converting their US-dollar-denominated assets into yuan assets?

assume u mean Chinese exporters convert the US dollar they earned from international trading into yuan.

Actually, as long as exporters live outside US and they are operating in the local area, they need to convert the US dollar they made into local currencies, whether living in China or Germany or Guyana.

Your employees can not use US dollar to pay their rents, utilities, gas, mortgage, lunch, flowers and cakes for their girlfriends birthday. So the company need to pay their staff, along with the company’s rents, utilities, gas, suppliers, in local currencies, coz the landlord, the power company, the gas station, the bank, the restaurant need to pay their expenses in local currency.