For our project we are using
val analyzer = new KeywordAnalyzer()
val luceneVersion = Version.parseLeniently(version)
Error CREATEing SolrCore 'ac_test2_shard2_replica1': Unable to create core [ac_test2_shard2_replica1] Caused by: Format version is not supported (resource: BufferedChecksumIndexInput(segments_1)): 6 (needs to be between 0 and 3)
Simply setting the version in the analyzer isn't going to do anything about the format of the index. All that does it make sure you are using familiar analysis rules, it has nothing to do with this problem.
You need to use the appropriate codec to write an index in an older format. Particularly
Lucene410Codec. You can set the codec to use in your
IndexWriterConfig. The backwards-codecs are primarily intended to read old indexes, rather than write them. I don't know for sure whether using it for your purpose would even work.
If possible, I would recommend you just use compatible Lucene versions instead. Either upgrade your Solr instance, or just use Lucene 4.10 for this job.