Changing a mapping without deleting the index

I changed a mapping template to add a "dynamic_date_formats" field, but
after restarting elasticsearch and flushing the index the change wasn't
picked up in my data.
The data type was only changed after I deleted and re-added the index. How
do I do this dynamically?

I added the template like this (the json is attached).
curl -XPUT http://localhost:9200/_template/couchbase -d