dch -D doesn't recognize unstable

its only a warning which must be acknowledged
the new entry will contain unstable when you press return.

--force-distribution disables the acknowledgment

None of which should be necessary.

Sorry if it's already solved (this bug is 1 year old), but this does work:

dch -v2.1.9-1 -Dunstable --distributor=Debian

This bug was fixed in the package devscripts - 2.11.8

---------------
devscripts (2.11.8) unstable; urgency=low

  [ David Prévot ]
  * French translation update.

  [ James McCoy ]
  * dd-list:
    + Recognize -h argument, as documented.
    + Don't error when given multiple binary packages from the same source.
      (Closes: #672309)
  * Also note DEBCHANGE_MAINTTRAILER change in NEWS entry for 2.11.7.
    (Closes: #672973)
  * dget:
    + Fix handling of sources.list entries with a port. (Closes: #672460) Still
      can't handle entries at the same domain but different ports until
      #154868 is fixed.
  * debcheckout:
    + Document the DEBCHECKOUT_SOURCE configuration variable.
    + Determine the source package name when downloading the source tarball.
      This ensures the downloaded files aren't incorrectly removed after being
      downloaded.
    + Adapt find_repo() to determine the tarball name for native packages.

  [ Benjamin Drung ]
  * debchange:
    + Add --vendor= and DEBCHANGE_VENDOR to override the distributor ID
      returned by dpkg-vendor.
    + Always perform Vendor check.
    + Fall back to Debian vendor when a Debian-specific command-line option
      has been supplied (--nmu, --qa, --bin-nmu, --bpo).
    + Adjust --security template for Ubuntu.
    + Add -R/--rebuild flag for Ubuntu's no-change rebuilds.
    + Append ubuntu1 to version when incrementing on Ubuntu, unless a
      -U/--upstream option is given.
    + On Ubuntu, don't copy the previous distribution name for a new changelog
      entry. Use the Ubuntu devel release.
    + Don't use NMU versioning for NMUs / Security uploads on Ubuntu.
    + dch --increment changes XbuildY to Xubuntu1 on Ubuntu (LP: #690230).
    + Try to guess the vendor based on the given distribution name (LP: #723715)
    + Prefer UBUMAIL over DEBEMAIL on Ubuntu (LP: #929846).
  * Add first tests for licensecheck.
  * Add online test for uscan.

  [ Stefano Rivera ]
  * devscripts.Logger Don't substitute arguments into logged strings unless
    they were provided. (LP: #968129)
  * debchange: Use distro-info to determine Ubuntu release names (LP: #997932).
  * Incorporate Ubuntu's delta:
    Move debian-keyring, equivs, libcrypt-ssleay-perl, and libsoap-lite-perl
    to Suggests when building on Ubuntu.

  [ Salvatore Bonaccorso ]
  * bts: When searching for usertags use tag= in the url (followed by
    the options containing users=). (Closes: #675071).

  [ Raphael Geissert ]
  * dget: ignore duplicate repository URLs. (Closes: #675258)

  [ Kees Cook ]
  * licensecheck: Catch LGPL more robustly. (Closes: #623283)

  [ Thijs Kinkhorst ]
  * debdiff: Do not generate warnings when debdiff'ing dpkg source format
    3.0 (git). (Closes: #668372)
  * debuild: Do not warn for missing upstream tarball if package is source
    format 3.0 (git). (Closes: #668372)

  [ Scott Moser ]
  * uscan: Support watch files that reference S3 bucket listings.
    (Closes: #630756, LP: #798293)

  [ Yaroslav Halchenko ]
  * licensecheck: Check licenses in .m (Octave/Matlab), .tex (LaTeX),
    and .pyx (Python's pyrex) files (Closes: #604529)

  [ Ivan Borzenkov ]
  * licensecheck: Add detection co...