They both stand for Zenithal Polynomial projections. The ZPN projection is now a standard projection as defined in Greisel & Calabretta (2002) and should be used wherever possible instead of ZPX. This projection is generally used to model camera distortions using polynomials and allows ...
IRAF still does not understand the standard ZPN projection, but it understands ZPX instead, so the solution is to transform the header WCS to its equivalent representation on ZPX projection. The solution is to run the next command on your sample.fits file:
hedit sample.fits 'CTYPE1' 'RA---ZPX' verify-) hedit sample.fits 'CTYPE2' 'DEC--ZPX' verify-) hedit sample.fits 'WAT1_001' \ 'wtype=zpx axtype=ra projp1=1.0 projp3=220.0' add+ verify- hedit sample.fits 'WAT2_001' \ 'wtype=zpx axtype=dec projp1=1.0 projp3=220.0' add+ verify-
You can access the old WFS pages following this link. Note, however that some information may not be updated.