Well, the first and obvious step is to make backups. But you’'ve already done that, right?
Next, Im fairly certain that there has been an upgrade path the whole way, though Ive never used JM 2.3. In the resources folder, there is a database/upgrade folder with a path going back to 1.1_to_2.0, but that all seems to be Wildfire.
I know there are old versions of wildfire out there for download (just no links too them), it may not be a bad idea to upgrade in steps, such as get to the first wildfire version, then upgrade wildfire. Hopfully someone who remembers using the older versions will chime in. It might be useful if you mention which OS and database you are using.
You may want to read the Steps for upgrading Jive Messenger to Wildfire[/url] document. Once you upgrade to Wildfire 2.4.0 the rest of the upgrades should be quite easy (just database changes - you can manually execute the upgrade db scripts). Let us know if you have more questions.