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/test/java/org/elasticsearch/cluster/metadata/DataStreamTests.java
ds.isReplicated(),
            ds.isSystem(),
            ds.isAllowCustomRouting(),
            indexMode
        );
        var newCoordinates = ds.nextWriteIndexAndGeneration(Metadata.EMPTY_METADATA);
        var rolledDs = ds.rollover(new Index(newCoordinates.v1(), UUIDs.randomBase64UUID()), newCoordinates.v2(), true);
        assertThat(rolledDs.getName(), equalTo(ds.getName()));
        assertThat(rolledDs.getTimeStampField(), equalTo(ds.getTimeStampField()));
        assertThat(rolledDs.getGeneration(), equalTo(ds.getGeneration() + 1));
        assertThat(rolledDs.getIndices().size(), equalTo(ds.getIndices().size() + 1));
        assertTrue(rolledDs.getIndices().containsAll(ds.getIndices()));
        assertTrue(rolledDs.getIndices().contains(rolledDs.getWriteIndex()));
        assertThat(rolledDs.getIndexMode(), equalTo(IndexMode.TIME_SERIES));
    }
    public void testRolloverIndexMode_keepIndexMode() {
        DataStream ds = DataStreamTestHelper.randomInstance().promoteDataStream();
        ds = new DataStream(
            ds.getName(),
            ds.getIndices(),
            ds.getGeneration(),
            ds.getMetadata(),
            ds.isHidden(),
            ds.isReplicated(),
            ds.isSystem(),
            ds.isAllowCustomRouting(),
            IndexMode.TIME_SERIES
        );
        var newCoordinates = ds.nextWriteIndexAndGeneration(Metadata.EMPTY_METADATA);
        var rolledDs = ds.rollover(new Index(newCoordinates.v1(), UUIDs.randomBase64UUID()), newCoordinates.v2(), false);