Important: also check the Frequently Asked Questions Supplement
The following Bacula versions are still in use today so it seems:
Besides various fileformatversions (2 versions as stated in a document from 2005), and various database layouts for the catalog database, also communications protocols changes have occured between releases. This leads to incompatible versions of components, component versions not intended to wrok together (but some people report that they do), however there are some rules of thumb:
(as abstracted from: http://bacula.org/en/?page=news)
Always check changelogs of releases between the two versions of Bacula you are planning to use
Recent changelogs are also published on: http://blog.bacula.org/category/releases/
It is highly recommended to keep Bacula components at the same versions