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
new Template(
                finalSettings.build(),
                finalTemplate.map(Template::mappings).orElse(null),
                finalTemplate.map(Template::aliases).orElse(null)
            ),
            indexTemplate.composedOf(),
            indexTemplate.priority(),
            indexTemplate.version(),
            indexTemplate.metadata(),
            indexTemplate.getDataStreamTemplate(),
            indexTemplate.getAllowAutoCreate()
        );
        validate(name, templateToValidate);
        validateDataStreamsStillReferenced(currentState, name, templateToValidate);
        validateTsdbDataStreamsReferringTsdbTemplate(currentState, name, templateToValidate);
        
        
        try {
            validateCompositeTemplate(currentState, name, templateToValidate, indicesService, xContentRegistry, systemIndices);
        } catch (Exception e) {
            throw new IllegalArgumentException(
                "composable template ["
                    + name
                    + "] template after composition "
                    + (indexTemplate.composedOf().size() > 0 ? "with component templates " + indexTemplate.composedOf() + " " : "")
                    + "is invalid",
                e
            );
        }
    }