Ok, after another careful trial and error session, I found the solution.
The jbpm/pom.xml file in the main project, requires as parent:
<parent>
<groupId>org.drools</groupId>
<artifactId>droolsjbpm-parent</artifactId>
<version>5.5.0.Final</version>
<!-- relativePath causes out-of-date problems on hudson slaves -->
<!--<relativePath>../droolsjbpm-build-bootstrap/pom.xml</relativePath>-->
</parent>
therefore I looked into .m2\repository\org\drools\droolsjbpm-parent\5.5.0.Final\droolsjbpm-parent-5.5.0.Final.pom, and here I found
<bpm.console.version>2.3.8.Final</bpm.console.version>
I changed this property, I compiled again, and everything worked fine.
PS: since this is my case, I might add that the .zip version number must be the same jBPM.version number specified inside the $jbpm-install-directory/build.properties file.
Also, you have to move the $source-directory/jbpm-form-builder/jbpm-form-builder-distribution/target/jbpm-5.4.3-SNAPSHOT-form-builder.war to the lib directory; the guide tells you to compile it at point 8, but the author must have forgotten to tell you to move the file. Once again, check that version numbers coincide, or the build file will automatically download another version and invalidate your changes.