The chances of the sleeper rail fare changing are diminishing small. They stay fixed at the second coach Value bucket. They are not yield managed. In sleepers, it is the accommodation charges that are subject to yield management and can shift quickly. Inventory shifts between buckets frequently, but the buckets themselves are much more stable.
An agent at a staffed station has the capability to create and ticket an open sleeper reservation as well as a regular eticket reservation. The only thing they cannot do is make an AGR points reservation.