Analyst upgrades typically lead to a positive short-term reaction in a stock's price as they suggest a bullish outlook. The upgrade from Neutral to Buy by a reputable firm like B of A Securities, coupled with a specific price target of $47, provides a clear positive signal to the market, indicating a higher level of confidence in Ternium's future performance. This can lead to increased investor interest and buying activity, driving up the stock price in the short term.