A system is provided for transforming an in-use RAID array from a first array configuration having a first parameter to a second array configuration having a second parameter while preserving a logical data structure of the RAID array. The system includes an extent reservation component, and a data migration component for reading unmigrated data from an area of an array arranged according to the first array configuration and writing the data to an area of the array arranged according to the second array configuration using reserved extents to store migrated data. The system also includes a first I/O component for performing I/O according to the first array configuration on unmigrated data prior to its reading by the data migration component, and a second I/O component for performing I/O according to the second array configuration on the migrated data after writing the migrated data.