1. A method of operating a memory system including a memory controller and a nonvolatile memory device, the method comprising: storing data in a buffer region of the nonvolatile memory device under the control of the memory controller; by operation of the memory controller, issuing a migration request directed to the data stored in the buffer region; executing a migration operation in the nonvolatile memory device in response to the migration request, where the migration operation moves the data from buffer region to a main region of the nonvolatile memory device; and upon completion of the migration operation, marking a migration operation completion time, and after an initial verify shift (IVS) time has elapsed following the migration operation completion time, updating a mapping table associated with the data in view of the executed migration operation.