Validation and verification of Software Product Lines is particularly challenging due to the complex structure and interaction of commonalities and variabilities among products. There are several approaches to specify the structure of such commonalities and variabilities, such as the delta-oriented approach. Building upon such a structure, we propose an approach to avoid redundant analysis in Software Product Lines by extending them to semantic behavioural changes. To this end, we propose to use Differential Symbolic Execution, an automated technique for proving functional behavioural equivalence based on satisfiability modulo theories. Our proposal aims at identifying the behavioural commonalities of one software product relative to another and exploits them in order to establish an efficient model-based testing trajectory.