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/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
final ClusterState state,
        final String templateName,
        final ComposableIndexTemplate template,
        final IndicesService indicesService,
        final NamedXContentRegistry xContentRegistry,
        final SystemIndices systemIndices
    ) throws Exception {
        final ClusterState stateWithTemplate = ClusterState.builder(state)
            .metadata(Metadata.builder(state.metadata()).put(templateName, template))
            .build();
        final String temporaryIndexName = "validate-template-" + UUIDs.randomBase64UUID().toLowerCase(Locale.ROOT);
        Settings resolvedSettings = resolveSettings(stateWithTemplate.metadata(), templateName);
        
        int dummyPartitionSize = IndexMetadata.INDEX_ROUTING_PARTITION_SIZE_SETTING.get(resolvedSettings);
        int dummyShards = resolvedSettings.getAsInt(
            IndexMetadata.SETTING_NUMBER_OF_SHARDS,
            dummyPartitionSize == 1 ? 1 : dummyPartitionSize + 1
        );
        int shardReplicas = resolvedSettings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0);
        
        Settings finalResolvedSettings = Settings.builder()
            .put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT)
            .put(resolvedSettings)
            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, dummyShards)
            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, shardReplicas)
            .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
            .build();
        
        final ClusterState stateWithIndex = ClusterState.builder(stateWithTemplate)
            .metadata(