Elasticsearch upgrade to 2.x and '.' issue in fieldname

Hi, I have a cluster which contains data from log files. The fields are dynamic.
With the breaking change that '.' could not be part of field name. I could fix this issue for new data.
But how should i update the existing fieldnames in mapping. Is their any utility which will take care of this. I think before making such a breaking change without any hack solution should have been their for making changes to existing data.

Any suggestion will save my day

There's no way to do this within ES.

I use Logstash to reindex - https://gist.github.com/markwalkom/8a7201e3f6ea4354ae06