How to use different similarity models (use DSL) in the process of search

Elastic Stack Elasticsearch
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  1. From the documentation, I know that I can use a different similarity model when building the index. The problem is my index is already built and very large, takes a few days to build. So, I wonder if it is possible to use a different similarity model (use DSL) when I do search.
  2. Though the documentation tells how to use a different similarity model with mapping API, I cannot find how to use DSL for a different model. Any codes?
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Are you referring to this - https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html?

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This reference talks about how to use similarity model APIs. But I want to embed codes into our program, i.e., use DSL.