Supporting the evolution of process model at run-time stage and propagating the changes of process model to the active process instances are fundamental requirements for any flexible process-aware information systems (PAIS). Instance migration is regarded as the mainstream technique that deals with the dynamic evolution of process-aware information systems. By using this technique, active process instances are migrated to the modified process model, so that they can benefit from the optimization process. The challenges are 1) to guarantee that the process instances can be correctly migrated to the modified process model; 2) to efficiently check whether the process instances can be migrated. From this view, this paper surveys the state-of-the-art dynamic evolution of processes in PAIS. Finally, some potential research directions on evolution of processes are proposed for future references.