Q: How do I use 'debdelta' to upgrade my Debian box, and (possibly)
   speedup/save on bandwidth?

A: Quick answer:
  apt-get update
  debdelta-upgrade
  apt-get upgrade


Q: Do you use 'xdelta' or 'bsdiff' ?

A: 'debdelta' has an option '--delta-algo' to choose between 
          xdelta xdelta-bzip xdelta3 bsdiff


Q: How do you integrate this approach with the minimal security
  that signed Release files give us today?

A: 'debpatch' recreates the new .deb , and guarantees that it is equal to the
  one in archives, so archive signatures can be verified
 (for this reason, 'debdelta'  does not use dpkg-repack ).
 Moreover, since version 0.31, delta files are GPG signed.


Q: What if any or both the debs are signed using dpkg-sig ?

A: Supported.


Q: What about .debs where the data part is compressed with bzip ?

A: supported since version 0.24


Q: And with lzma?

A: Since version 0.31


Q: can 'debpatch' recreate the new .deb using the installed old .deb, even when
   there are dpkg-diversions ?

A:  yes.


Q: can 'debpatch' recreate the new .deb using the installed old .deb, even when
   conf files were modified ?

A:  yes.


Q: can 'debpatch' recreate the new .deb using the installed old .deb, 
  when 'prelink' is used in the host?

A: since version 0.27


Q: How does 'debdelta' work? What is the content of delta files?

A: See the example script /usr/share/debdelta/debpatch.sh


Q: What about backward compatibility? If I create a delta with the
   most recent version of 'debdelta' will another user be able to
   apply it with an older version of 'debpatch' ?

A: It depends. A delta contains some keywords in the form
  'need-xxxx' that express the requirements for applying it, so that
  'debpatch' will graciously fail if it sees that it cannot satisfy
  the requirements, and ask to be upgraded.


Q: What about backward compatibility, the other way round? If I created
   a delta long ago with a very old version of 'debdelta', will I be 
   be able to apply it with a future version of 'debpatch' ?

A: This should always be possible (but for bugs).