Hi Bart, Jeen,
it is also my understanding that a new major version of Lucene (and others) can read the indices of the previous version, for version jumps this is not guaranteed. I vaguely remember that for some lucene version even two major versions were supported, however, in general the best practice is to re-index when upgrading to a new major version.
For the Lucene case I will see compatibilty when I do tests in our platform.
Regarding my change: from my point of view the upgrade on the code level is fine, all integration tests are working locally.
However, I have had a hard time of getting the ElasticSearch Integration test to run: it always complains a JarHell. I tried to follow the path (and resolved quite some inconsistencies), but this is a never ending story with all the load of dependencies. For example it complains with mockito and securemock being on the test classpath at the same time, the same for logging frameworks, hamcrest and its dependencies, http client minor versions (which are easy to resolve).
(i.e. overriding JarHell on the classpath with noops, and disabling the security manager). With this I could successfully run all ElasticSearch Compliance tests in Eclipse.