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
}
        Metadata.Builder builder = Metadata.builder();
        for (int k = 1; k <= numBackingIndices; k++) {
            IndexMetadata im = IndexMetadata.builder(indices.get(k - 1).getName())
                .settings(settings(Version.CURRENT))
                .numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            builder.put(im, false);
        }
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        builder.put(original);
        Index indexToAdd = new Index(randomAlphaOfLength(4), UUIDs.randomBase64UUID(random()));
        IndexMetadata.Builder b = IndexMetadata.builder(indexToAdd.getName())
            .settings(settings(Version.CURRENT))
            .numberOfShards(1)
            .numberOfReplicas(1);
        final int numAliases = randomIntBetween(1, 3);
        final String[] aliasNames = new String[numAliases];
        for (int k = 0; k < numAliases; k++) {
            aliasNames[k] = randomAlphaOfLength(6);
            b.putAlias(AliasMetadata.builder(aliasNames[k]));
        }
        builder.put(b.build(), false);
        Arrays.sort(aliasNames);
        IllegalArgumentException e = expectThrows(
            IllegalArgumentException.class,
            () -> original.addBackingIndex(builder.build(), indexToAdd)
        );
        assertThat(