Multiterm hasn't been modified a lot on the database level between 2007 and 2009. The compatibility between these 2 versions is not as limited as it seems: file extensions are different but contents are very similar.
SDL documented how to upgrade a 2007 termbase. Here the way to downgrade a 2009 termbase. It's not that I prefere 2007, just that Studio is not to be found on every translator's computer.
2009 server termbases have to be exported (use the default filter, you get an xml file) and imported on the 2007 server. The definition file of 2009 (xdt file) can be used on 2007 without any modification.
To downgrade local termbases, it's much easier. Just change the extension from "sdltb" to "mdb"!
In Multiterm 2007, use "Load External Termbase..." in "Termbase" to load it in your Multiterm Desktop 2007.