The Replacements are a band with an off-the-charts cool factor, a trove of indelible songs and the potential to draw from an expanding fan base. They’ll not only attract many of the original followers nostalgic for the ’80s but also subsequent generations eager to see a legendary band perform live for the first time. When last seen more than 20 years ago, the Replacements were selling out 4,000-seat theaters. By then, founding members Bob Stinson and Chris Mars were long gone, and only Westerberg and Tommy Stinson remained from the original lineup. The ramshackle charm of the early days had long since been replaced by a steady professionalism, until that one final shot of chaos in Grant Park.