1. A neurostimulation system, comprising: an implantable device connected to at least two implantable neurostimulation leads, the implantable device including a neurostimulator configured to deliver, using a first set of stimulation parameters, electrical energy through the neurostimulation leads to spinal cord tissue; and an external device configured to communicate with the implantable device, wherein the neurostimulation system includes: monitoring circuitry configured to monitor migration between the leads and monitor migration of at least one of the leads with respect to the spinal cord tissue; and at least one controller/processor configured to communicate with the monitoring circuitry, to use the monitored migration between the leads, the monitored migration of at least one of the leads with respect to the spinal cord tissue, and at least one threshold value to determine when lead migration is significant, and to generate, when lead migration is determined to be significant, a second set of stimulation parameters using the monitored migration between the leads and the monitored migration of at least one of the leads with respect to the spinal cord tissue, wherein the neurostimulator is configured to use the first set of stimulation parameters to stimulate a virtual multipole defining a volume of activation relative to the spinal cord tissue, and to use the second set of stimulation parameters, which was generated using the monitored migration between the leads and the monitored migration of at least one of the leads with respect to the spinal cord tissue, to stimulate the virtual multipole wherein the second set of stimulation parameters include fractionalized amplitude values that emulate the virtual multipole.