You should now use fancypagestyle to set all page headers/footers and corner marks, like this:

\makeatletter
  \fancypagestyle{AMCpageFull}{
  \fancyhead[L]{\AMC@LR{\he@dbas{\leavevmode\m@rque{positionHG}}}}%
  \fancyhead[R]{\AMC@LR{\he@dbas{\leavevmode\m@rque{positionHD}}}}%
  \fancyfoot[L]{\AMC@LR{\leavevmode\m@rque{positionBG}}}%
  \fancyfoot[R]{\AMC@LR{\leavevmode\m@rque{positionBD}}}%
  \fancyhead[C]{ ...header... }
  \fancyfoot[C]{\AMCIDBoxesABC}
}
\makeatother

AMCpageFull is used for a standard page, AMCpageHeadOnly for a page in the corrected sheets, AMCpageNoMarks for a page with no marks.