A nested optimization strategy based on task assignment and Particle Swarm Optimization (PSO) is proposed to solve the optimization problem of formation transformation. Both the corresponding assigned target positions of interchangeable UAVs from initial formation to target formation and the relative position relationship between two formations are investigated in this paper. The Hungarian algorithm is used to rapidly solve the assignment problem, while the PSO is adopted to compute the optimal relative position relationship iteratively. To demonstrate the effectiveness and the universality of the proposed strategy, simulation results considered different situations are presented.