A flexible and correct model of the activity flows is required for workflows in product development environments. In particular, the design activity flows are not known until run-time, and conventional approaches have limit to handle this situation because they cannot predefine all the potentially reachable paths. Thus, the structure of the workflow model must be flexible enough to describe variety in workflow design and accommodate dynamic changes during workflow execution. In this paper, we provide the general primitive axioms and change patterns based on event calculus for dynamic workflow specification and execution mechanisms in product development environments. Also, we describe how to execute the workflow dynamically based on the workflow specification and workflow change patterns using abductive planning technique.