By Ilaina Jonas

NEW YORK, June 26 (Reuters) - Standard & Poor's Ratings Services on Friday placed 1,584 U.S. commercial mortgage-backed securities (CMBS) tranches from 209 deals on CreditWatch with negative implications, after issuing its refined ratings methodology.

The securities on CreditWatch have a par balance of $235.2 billion, S&P said.

The ratings agency also affirmed its triple-A ratings on 1,394 tranches from 340 transactions with a par balance of $249.8 billion.

The changes had little impact on the market on Friday because S&P didn't describe how they would affect downgrades, J.P. Morgan analyst Alan Todd said in an e-mail.

Last month, S&P said it would shift the methodology it used to rate CMBS bonds that would likely result in downgrades to 90 percent of the best commercial mortgage-backed securities made in 2007, the peak of the U.S. commercial real estate market.

S&P at the time said the changes also could affect about 25 percent of the most senior slices of CMBS issued in 2005 and 60 percent of the most senior issued in 2006.

The years 2005 through 2007 are seen as the top of the U.S. commercial real estate boom that was fed by loose underwriting standards.

That announcement, which shook the CMBS market, came a week after the U.S. Federal Reserve announced its revised plan to help get the moribund CMBS market restarted.