The CASA task exportfits sets the header keyword BUNIT as 'JY/BEAM' in CASA 4.2 and earlier, whereas it is 'Jy/beam' in CASA >=4.3. The BUNIT keyword is case-sensitive and 'Jy/beam' is adopted as FITS standard.
This change of the units can cause a problem on the beam-area conversion with some AIPS tasks if the AIPS version is <= 31DEC14 and the CASA version for exportfits is >=4.3. The issue has been fixed in the 31DEC15 version of AIPS. The solution for older versions of AIPS is to change the units in the header to 'JY/BEAM' from 'Jy/beam' with PUTHEAD.