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When I uprade Lucene, for example from 8.8.2 to 9.0.0, do I have to reindex?

NoNot necessarily, you can add a version specific lucene-backward-codecs library (, for example lucene-backward-codecs-9.0.0.jar ) and Lucene will be able will enable Lucene 9 to use the index of a the previous version Lucene 8.

But it is recommended to upgrade, because a Lucene 8.x index may not be able to be read by an eventual Lucene 10 release.

Certain changes always Nevertheless, certain changes will require to reindex though, also see https://solr.apache.org/guide/8_0/reindexing.html

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