on updates, either merge or replace the existing field data.

[agnellvj]

In certain situations, one may want to do merge the existing fields with the new data and store the combined result.

I was thinking something like this would be useful as the JSON passed to ES.
"merge" is the flag to say if you want to combine fields. I think for bulk updates the default should be true.

{"search" : {QUERY}, "merge" : "false", "data" : { "tags" : ["hot", "confidential"] } }