Complex equipment usually needs to be designed according to a working environment, for example Tunnel Boring Machine (TBM). But the environment can change in an unanticipated way, which usually leads to inferior performance for equipment and ultimately results in a great loss. Thus how to rapidly respond to unanticipated environment changes (UEC) is an important issue to be resolved for existing products. This paper discusses the adaptable redesign of existing equipment, which performs adaptation or adding new components in existing equipment, avoids redesigning or remanufacturing new equipment so as to quickly meet expected performance requirements. For a complex equipment with many interrelated parameters, there might exist couplings and even contradictions in the redesign. And it is difficult for engineers to determine how changes in certain parameters affect the system performance. The key issues of the redesign are discussed including function-structure analysis, structure change propagation prediction and contradiction resolving. The design issue is analyzed in an existing TBM responding to unanticipated ground changes.