Requirements evolve during the software development process. Requirements specification evolution determines changes both in terms of level of details and style of representation and it brings the requirements from the initial statement of the customer needs towards more detailed documents able to drive the software design, construction and verification. Controlling the evolution of requirements, usually written in natural language, is very important for the success of the whole software project because every step in the evolutionary path of requirements can introduce undesired changes or lacks of information. This paper describes an approach based on the Formal Concepts Analysis that allows a systematic and precise verification of the consistency among different stages of natural language requirements evolution.