User Exit that triggers for Change operation

Hi All..

I want a user exit which triggers when I enter into an infotype(2001) from tcode PA30.

My actual requirement is to show the field REFNR in display mode.

By using the exit PBAS0001 exit0001 I can make the field REFNR in displaymode for create operation.

But in change operation of the infotype the exit is not getting triggered.

Regards,

Anji.