1. A system comprising: a plurality of sources of memory operations, where each source is configured to transmit memory operations and corresponding quality of service (QoS) parameters indicating a requested QoS for the corresponding memory operations; and a memory controller coupled to the plurality of sources and configured to control access to a memory for the memory operations, wherein the memory controller comprises a plurality of queues configured to store memory operations, and wherein the memory controller is configured to arbitrate among the memory operations responsive to the QoS parameters, and wherein the memory controller is coupled to receive an upgrade QoS indication from a first source of the plurality of sources, and wherein the memory controller is configured to modify the QoS parameters of pending memory operations in the queues, and wherein the pending memory operations were received from the first source, and wherein the QoS parameters are modified responsive to receiving the upgrade QoS indication from the first source subsequent to receiving the pending memory operations, wherein the modified QoS parameter for a given pending memory operation is different from an original QoS parameter provided by the first source with the pending memory operation at a time that the pending memory operation is received from the first source, and wherein the modified QoS parameter indicates a higher level of QoS than a level of QoS indicated by the original QoS parameter provided with the pending memory operation.