Why relevance score becomes so high in ES 5.2?

[Youxu (Xudong You)]

In our old ES 1.x & 2.x cluster, the max relevance score is generally less than 3, but on ES 5.2, I saw very high relevance score that greater than 200.

Is it because the default similarity algorithm changed to BM 25 on ES 5.2? Is there a easy way to normalize the score to be similar to the score in ES 1.x & 2.x?

[softwaredoug (Doug Turnbull)]

This is because the default similarity is now BM25. The scores seem to tend
higher. Generally it's nothing to be concerned about

More here

BM25 The Next Generation of Lucene Relevance

You can change back to the classic TF*IDF score here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html

But I'd ask yourself why? BM25 is generally a better general purpose
scoring solution based on information retrieval research