As of this writing (March 2011), all data in the IRS archive have been reprocessed with version S18.18 of the IRS pipeline.
CUBISM version 1.8 defaults to a calibration set corresponding to pipeline version S18.18.
If you downloaded your data and built cubes some time ago with an older version of CUBISM, then the calibration set you used is saved in the CUBISM cpj file. Therefore, you are free to use CUBISM version 1.8 in order to access its new features without compromising your calibration. However, you should check the IRS pipeline history log in the IRS Instrument Handbook to see if there have been changes which could affect your science. In that case, you should download your newly-processed data from the archive and use the default calibration in CUBISM version 1.8 .