Diet Pepsi is tweaking its formula to stay sweet a little longer. PepsiCo Inc. is testing a new mix of artificial sweeteners that lets the soda keep its taste for a longer period of time. The problem is that the current sweetener used in the soda - aspartame - loses its potency faster than high fructose corn syrup, the sweetener that's used in most regular sodas. A person with knowledge of the situation who requested anonymity because she wasn't authorized to speak publicly says the company had considered importing versions of Diet Pepsi sold in other countries to the U.S. But now it's testing a new version that could come out next year. The new version uses a mix of artificial sweeteners, according to the person.