Flexible capacity reservations for network-accessible resources

Methods and apparatus for flexible capacity pool reservations for network-accessible resources are disclosed. A system includes a plurality of resources of a provider network, and a resource manager. The resource manager receives a reservation modification request from a client that has an existing reservation for an original resource capacity and an original reservation duration. The modification request specifies a desired resource capacity. The reservation manager identifies resources to provide the desired capacity, and determines pricing components for the requested modification based on the difference between the desires resource capacity and the original capacity and on the remaining portion of the duration of the existing reservation. The resource manager modifies the reservation as requested and notifies the client regarding the pricing components.