https://lucene.apache.org/core/4_10_3/core/org/apache/lucene/index/class-use/IndexUpgrader.html
Uses of Class org.apache.lucene.index.IndexUpgrader (Lucene 4.10.3 API)
apache lucene
https://lucene.apache.org/core/5_5_0/core/org/apache/lucene/index/IndexUpgrader.html
IndexUpgrader (Lucene 5.5.0 API)
luceneapi