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
}
        var indicesToAdd = DataStreamTestHelper.randomIndexInstances();
        var postSnapshotIndices = new ArrayList<>(preSnapshotDataStream.getIndices());
        postSnapshotIndices.removeAll(indicesToRemove);
        postSnapshotIndices.addAll(indicesToAdd);
        var postSnapshotDataStream = new DataStream(
            preSnapshotDataStream.getName(),
            postSnapshotIndices,
            preSnapshotDataStream.getGeneration() + randomIntBetween(0, 5),
            preSnapshotDataStream.getMetadata() == null ? null : new HashMap<>(preSnapshotDataStream.getMetadata()),
            preSnapshotDataStream.isHidden(),
            preSnapshotDataStream.isReplicated() && randomBoolean(),
            preSnapshotDataStream.isSystem(),
            preSnapshotDataStream.isAllowCustomRouting(),
            preSnapshotDataStream.getIndexMode()
        );
        var reconciledDataStream = postSnapshotDataStream.snapshot(
            preSnapshotDataStream.getIndices().stream().map(Index::getName).toList()
        );
        assertThat(reconciledDataStream.getName(), equalTo(postSnapshotDataStream.getName()));
        assertThat(reconciledDataStream.getTimeStampField(), equalTo(postSnapshotDataStream.getTimeStampField()));
        assertThat(reconciledDataStream.getGeneration(), equalTo(postSnapshotDataStream.getGeneration()));
        if (reconciledDataStream.getMetadata() != null) {
            assertThat(
                new HashSet<>(reconciledDataStream.getMetadata().entrySet()),
                hasItems(postSnapshotDataStream.getMetadata().entrySet().toArray())
            );
        } else {
            assertNull(postSnapshotDataStream.getMetadata());
        }