Continuum 1.2.4 has not yet been released, so links to it in the central repo won't work. Sorry about that... the lack of a smooth upgrade path is one reason that 1.3.1 is only an Alpha release.
If you'd like to try the upgrade to 1.3.1 Alpha, you can build the 1.2.x branch from svn:
svn co http://svn.apache.org/repos/asf/continuum/branches/continuum-1.2.x/
You will find the data management jar in continuum-data-management/data-management-cli/target .
I've also deployed a new snapshot of the 1.2.x branch to the ASF snapshots repo. You can try that if you prefer not to build it yourself. (Grab the data management cli jar, then add this repo to your settings.xml so it can find the rest of the things it needs.)