Fuctionalities in a software system can be categorized as accidental or essential, where accidental functionalities are dependent on essential ones, but not vise vesa. Distinguishing essential functionalities from accidental ones in the development can lead to the reduction of maintenance effort and to the trustworthy system. In this paper, we propose Feature Based Mobility (FBM) in which accidental functionalities are aligned with essential functionalities. The feature in this approach, as a logical unit, provides traceability from a functionality in the requirements to an implemented module with effective dependency management, thus the accidental features can be modified with minimal disruption to the essential features. FBM enables us to limit working boundary of maintenance activities, thus helps to keep the trustworthiness of the system against unskillful modification.