Unlike DecompressAll which is designed merely for performance improvement, this method permanently changes the data as if written out in an uncompressed transfer syntax and then read back. This might therefore change the Photometric interpretation etc. (e.g. from YBR_FULL_422 to RGB)
DicomDataSet ds = new DicomDataSet("Compressed_DataSet.dcm");
ds.ChangeToDecompressed();
// Dataset now has uncompressed pixel data - optionally write modified ds to new file
ds.Write("Decompressed_DataSet.dcm");