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
+ name
                                + "] results in invalid composable template ["
                                + composableTemplateName
                                + "] after templates are merged",
                            e
                        );
                    } else {
                        validationFailure.addSuppressed(e);
                    }
                }
            }
            if (validationFailure != null) {
                throw validationFailure;
            }
        }
        logger.info("{} component template [{}]", existing == null ? "adding" : "updating", name);
        return ClusterState.builder(currentState)
            .metadata(Metadata.builder(currentState.metadata()).put(name, finalComponentTemplate))
            .build();
    }
    @Nullable
    private static CompressedXContent wrapMappingsIfNecessary(@Nullable CompressedXContent mappings, NamedXContentRegistry xContentRegistry)
        throws IOException {
        
        
        CompressedXContent wrapped = mappings;
        if (wrapped != null) {
            Map<String, Object> parsedMappings = MapperService.parseMapping(xContentRegistry, mappings);
            if (parsedMappings.size() > 0) {
                if (parsedMappings.size() == 1) {