Currently it is possible to migrate an existing data stream to a time series data stream.
This is done by updating the index template that created the data stream. 
Then after a rollover the data stream is a time series data stream. 

Via a similar way it should be possible to change time series data stream back 
to a regular data stream. Currently when changing the `index.mode` setting
on an index template from `time_series` to `standard` (or removing the setting)
results in a validation error.
server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
}
        }
        return overlappingTemplates;
    }
    /**
     * Return a map of v2 template names to their index patterns for v2 templates that would overlap
     * with the given template's index patterns.
     */
    public static Map<String, List<String>> findConflictingV2Templates(
        final ClusterState state,
        final String candidateName,
        final List<String> indexPatterns
    ) {
        return findConflictingV2Templates(state, candidateName, indexPatterns, false, 0L);
    }
    /**
     * Return a map of v2 template names to their index patterns for v2 templates that would overlap
     * with the given template's index patterns.
     *
     * Based on the provided checkPriority and priority parameters this aims to report the overlapping
     * index templates regardless of the priority (ie. checkPriority == false) or otherwise overlapping
     * templates with the same priority as the given priority parameter (this is useful when trying to
     * add a new template, as we don't support multiple overlapping, from an index pattern perspective,
     * index templates with the same priority).
     */
    static Map<String, List<String>> findConflictingV2Templates(
        final ClusterState state,
        final String candidateName,
        final List<String> indexPatterns,
        boolean checkPriority,