[arafalov]

The primary limitation comes from Lucene, which powers both. With Solr, you could change the definition and reload the core, but you will be getting some weird artifacts for the previously-indexed content.

Neither Solr nor Elasticsearch should be treated as a primary data store, so that's probably why reindex-in-place is not the highest priority.

[lobster_johnson]

I think treating Solr/ES as a primary data store would be a horrible idea. But the problem is equally annoying if you're using it as an expendable search index.

There is no technical reason why Solr/ES could not do diff-based indexing. ES (I don't know about Solr) admittedly uses a single Lucene index per logical index, so changing a single field mapping involves reindexing the whole index, not just that one mapping.

But if the mappings were properly versioned ES could simply create a new version, index everything (from the original contents), and then swap. Locking the original index should be a non-issue.