Many UAV-related applications require group communications between UAVs to reliably and efficiently deliver rich media content as well as to extend line-of-sight coverage between sky and ground. This paper is among the first to study fast yet resource-efficient UAV transitions for aerial group communications while maintaining high group communication performance. We develop a set of novel analytic and algorithmic results to form the efficient transition formation (ETF) algorithm, in order to deal with different UAV transition scenarios in a group communication environment. The ETF algorithm first evaluates the seamlessness of a straight-line trajectory (SLT) by processing low-complexity computations (e.g., Euclidean distances) or fast checks with controlled traffic overheads. For a non-seamless SLT, the ETF algorithmm establishes a new trajectory consisting of a minimal number of seamless straight lines that join at specially selected locations to control mobile UAVs’ seamless travel distances. Our simulation studies quantify the performance gains that the ETF algorithm may achieve, outperforming compared studies by admitting 66% more traffic with reduced energy consumption and guaranteed communication performance for both transitional and non-transitional UAVs.