This work first explores using flexible beam-user mapping to optimize the beam service range and beam position, in order to adapt the non-uniform traffic demand to offer in high-throughput satellite systems. Second, on this basis, the joint flexible bandwidth allocation is adopted to adapt the offer to demand at the same time. This strategy allows both beam capacity and load to be adjusted to cope with the traffic demand. The new information generated during the load transfer process of flexible beam-user mapping can guide the direction of beam optimization. Then, the proposed strategies are tested against joint power-bandwidth allocation and joint optimization of bandwidth and beam-user mapping under different traffic profiles. Numerical results are obtained for various non-uniform traffic distributions to evaluate the performance of the solutions. Results show that flexible joint load and capacity scheduling are superior to other strategies in terms of demand satisfaction with acceptable complexity.