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/DataStream.java
builder.startObject();
        builder.field(NAME_FIELD.getPreferredName(), name);
        builder.field(TIMESTAMP_FIELD_FIELD.getPreferredName(), TIMESTAMP_FIELD);
        builder.xContentList(INDICES_FIELD.getPreferredName(), indices);
        builder.field(GENERATION_FIELD.getPreferredName(), generation);
        if (metadata != null) {
            builder.field(METADATA_FIELD.getPreferredName(), metadata);
        }
        builder.field(HIDDEN_FIELD.getPreferredName(), hidden);
        builder.field(REPLICATED_FIELD.getPreferredName(), replicated);
        builder.field(SYSTEM_FIELD.getPreferredName(), system);
        builder.field(ALLOW_CUSTOM_ROUTING.getPreferredName(), allowCustomRouting);
        if (indexMode != null) {
            builder.field(INDEX_MODE.getPreferredName(), indexMode);
        }
        builder.endObject();
        return builder;
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        DataStream that = (DataStream) o;
        return name.equals(that.name)
            && indices.equals(that.indices)
            && generation == that.generation
            && Objects.equals(metadata, that.metadata)
            && hidden == that.hidden
            && replicated == that.replicated
            && allowCustomRouting == that.allowCustomRouting