Change a pgm image graphically and save without losing the original header
I re-send this question because it was misunderstood. I want to open a
pgm file with GIMP and "play" with its image graphically. Then I want
to save the modified image. While doing so I want that the original
header (particularly the comment section that start with the mark "#")
will be saved with the modified image/file. Currently the modified file
is saved with a default header that is put in by GIMP and the original
comment in the original header is lost.
Can I do it and how?