Are the occupation (OCC1990) and occupational standing (HWSEI PRENT) variables comparable over time (1990s-2013)?

The occupational codes changed across Censuses. How did IPUMS ensure comparability of occupation (OCC1990) and occupational standing variables (HWSEI PRENT) across years (1990s-2013)?
Thank you very much!

See Integrated Occupation and Industry Codes and Occupational Standing Variables in the IPUMS.