SHANGHAI, Sept 16 (Reuters) - China Evergrande Group’s main unit Hengda Real Estate Group Co Ltd said on Thursday it had applied to suspend trading of its onshore corporate bonds for one day and would change the trading mechanism on the resumption of trade, citing a downgrade. Hengda had received notice on Sept. 15 from rating agency China Chengxin International (CCXI) that its bonds’ ratings had been downgraded to A from AA, and that both the bonds ratings and its issuer ratings were put on a watch list for further downgrades, it said in a statement. Trade in its Shanghai exchange-traded bonds by auction would be suspended until further notice from Sept. 16, and would only be traded through negotiations.