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.
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
assertOK(response);
        }
        {
            var putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
            putComposableIndexTemplateRequest.setJsonEntity(NON_TSDB_TEMPLATE);
            var e = expectThrows(ResponseException.class, () -> client().performRequest(putComposableIndexTemplateRequest));
            assertThat(
                e.getMessage(),
                containsString(
                    "composable template [1] with index patterns [k8s*], priority [null],"
                        + " index.routing_path [] would cause tsdb data streams [k8s] to no longer match a data stream template"
                        + " with a time_series index_mode"
                )
            );
        }
    }
    public void testUpdateComponentTemplateDoesNotFailIndexTemplateValidation() throws IOException {
        var request = new Request("POST", "/_component_template/custom_template");
        request.setJsonEntity("""
            {
                "template": {
                    "settings": {
                        "index": {
                            "number_of_replicas": 1
                        }
                    }
                }
            }
            """);
        client().performRequest(request);