I'd say it might not that simple to do it.
You also need update bundle versions which karaf 2.2 depend on, besides it, not exactly sure other bundles which on top of karaf 2.1.x can work with karaf 2.2 without any problems.
As Freeman indicated, it might not be so easy to upgrade to Karaf 2.2.x on our existing 4.3.1-fuse release. However, the good news is that we're also working towards a new 4.4.0-fuse release. The latest snapshot are still unstable, but we should have usable snapshots in a few days. You can keep an eye on the snapshots in http://repo.fusesource.com/nexus/content/repositories/snapshots/org/apache/servicemix/apache-servicemix/4.4.0-fuse-SNAPSHOT/.
Thanks! That answered my question!