Should I reindex documents in elasticsearch when I update settings of index

Question

I've read the re-indexing documents which is required if anything has been changed in the existing index then I've to re-index documents. For example. If I make a change in the existing field then I've to reindex all the documents.

Should I re-index documents if I make a change in settings of an existing index?

What if I want to add a new analyzer/filter in the settings? Does it require to re-index documents?

Answer 1

not necessarily, for example, changing shard number definitely requires a reindex, b/c ES needs to reshard all document. changing replicas doesn't need reindex, and in fact a common practice to speed up your index with replica = 0, then later turns it = 1.