Package pdftk has been replaced by pdftk-java. Thus in the upgrade
from the aptitude curses UI, pdftk was selected as upgraded and
pdftk-java was selected as newly installed (as a dependency of
pdftk). Since I did not want to keep the transitional dummy package
pdftk, I marked pdftk-java as manually installed by typing 'm'
(thus the 'A' disappeared as expected) and marked pdftk to be
purged by typing '_'. Then I did the upgrade with 'gg'.

But after quitting aptitude, "apt install -f" said:

The following packages were automatically installed and are no longer required:
  libcommons-lang3-java pdftk-java
Use 'apt autoremove' to remove them.

and when I ran aptitude again, aptitude proposed to remove pdftk-java.