Key TakeawaysAmerican Express (Amex) has "limited incremental upside," according to Bank of America Securities (BofA) analysts.The investment firm downgraded Amex stock to "neutral" from "buy," while retaining a price target of $263 a share.Analysts warned of Amex facing a challenging spending environment, even among high-end consumers. American Express (AXP) shares traded lower Wednesday after Bank of America Securities analysts raised concerns that the stock was trading at a premium without too much poten ...