Changes are inevitable during software development and so are their unintentional side effects. The focus of this paper is on UML design models, where unintentional side effects lead to inconsistencies. We demonstrate that a tool can assist the designer in discovering unintentional side effects, locating choices for fixing inconsistencies, and then in changing the design model. Our techniques are "on-line," applied as the designer works, and non-intrusive, without overwhelming the designer. This is a significant improvement over the state-of-the-art. Our tool is fully integrated with the design tool IBM Rational Rose^TM. It was empirically evaluated on 48 case studies.