As part of a refactoring project I'm moving our quering end to ElasticSearch. Goal is to refactor the indexing-end to ES as well in the end, but this is pretty involved and the indexing part is running stable so this has less priority.
This leads to a situation where a Lucene index is created / indexed using Solr and queried using Elasticsearch. To my understanding this should be possible since ES and SOlR both create Lucene-compatable indexes.
Just to be sure, besides some housekeeping in ES to point to the correct index, is there any unforseen trouble I should be aware of when doing this?
You are correct, Lucene index is part of elasticsearch index. However, you need to consider that elasticsearch index also contains elasticsearch-specific index metadata, which will have to be recreated. The most tricky part of the metadata is mapping that will have to be precisely matched to Solr schema for all fields that you care about, and it might not be easy for some data types. Moreover, elasticsearch expects to find certain internal fields in the index. For example, it wouldn't be able to function without _uid field indexed and stored for every record.
At the end, even if you will overcome all these hurdles you might end up with fairly brittle solution and you will not be able to take advantage of many advanced elasticsearch features. I would suggest looking into migrating indexing portion first.
Have you seen ElasticSearch Mock Solr Plugin? I think it might help you in the migration process.