Changes in requirements may have a severe impact on development processes. For example, if requirements change during the course of a software development activity, it may be necessary to reschedule development activities so that the new requirements can be addressed in a timely manner. Unfortunately, current software development methods do not provide explicit means to adapt development processes with respect to changes in requirements. The paper proposes a method based on Markov decision theory, which determines the estimated optimal development schedule with respect to probabilistic product demands and resource constraints. This method is supported by a tool and applied to an industrial case.