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/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java
TimeValue lookAheadTime = TimeValue.timeValueHours(2); 
        Settings settings = Settings.EMPTY;
        String mapping = """
            {
                "_doc": {
                    "properties": {
                        "field1": {
                            "type": "long"
                        },
                        "field2": {
                            "type": "keyword"
                        },
                        "field3": {
                            "type": "keyword",
                            "time_series_dimension": true
                        }
                    }
                }
            }
            """;
        Settings result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,
            now,
            settings,
            List.of(new CompressedXContent(mapping))
        );
        assertThat(result.size(), equalTo(3));