1. Yardeni shifts advice from U.S. exceptionalism to recognizing global technology impacts. 2. He advises reducing holdings in Magnificent Seven to match S&P 500 weightings. 3. Tech and communication sectors make up 45% of the S&P 500 market value. 4. U.S. stocks show expensive valuations compared to international peers this year. 5. Yardeni forecasts competition will erode profits of large tech companies.