To compare the upcoming version of Central (${version}) against an older version, add lines similar to these your eclipse.ini file after the -vmargs line for the appropriate version & URLs:
 -Djboss.discovery.directory.url=http://download.jboss.org/jbosstools/discovery/development/${version}/jbosstools-directory.xml
 -Djboss.discovery.site.url=http://download.jboss.org/jbosstools/discovery/development/${version}/
 -Djboss.discovery.earlyaccess.site.url=http://download.jboss.org/jbosstools/discovery/development/${version}/
"
fi