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
final String keyName = parsedMappings.keySet().iterator().next();
                    
                    if (MapperService.SINGLE_MAPPING_NAME.equals(keyName) == false) {
                        wrapped = new CompressedXContent(
                            (builder, params) -> builder.field(MapperService.SINGLE_MAPPING_NAME, parsedMappings)
                        );
                    }
                } else {
                    wrapped = new CompressedXContent((builder, params) -> builder.field(MapperService.SINGLE_MAPPING_NAME, parsedMappings));
                }
            }
        }
        return wrapped;
    }
    /**
     * Remove the given component template from the cluster state. The component template name
     * supports simple regex wildcards for removing multiple component templates at a time.
     */
    public void removeComponentTemplate(
        final String[] names,
        final TimeValue masterTimeout,
        ClusterState state,
        final ActionListener<AcknowledgedResponse> listener
    ) {
        validateNotInUse(state.metadata(), names);
        clusterService.submitStateUpdateTask(
            "remove-component-template [" + String.join(",", names) + "]",
            new TemplateClusterStateUpdateTask(listener) {
                @Override
                public ClusterState execute(ClusterState currentState) {