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
private static Alias toAlias(AliasMetadata aliasMeta) {
        Alias a = new Alias(aliasMeta.alias());
        if (aliasMeta.filter() != null) {
            a.filter(aliasMeta.filter().string());
        }
        a.searchRouting(aliasMeta.searchRouting());
        a.indexRouting(aliasMeta.indexRouting());
        a.isHidden(aliasMeta.isHidden());
        a.writeIndex(aliasMeta.writeIndex());
        return a;
    }
    private void validate(PutRequest putRequest) {
        validate(putRequest.name, putRequest.settings, putRequest.indexPatterns, putRequest.aliases);
    }
    private void validate(String name, @Nullable Settings settings, List<String> indexPatterns, List<Alias> aliases) {
        List<String> validationErrors = new ArrayList<>();
        if (name.contains(" ")) {
            validationErrors.add("name must not contain a space");
        }
        if (name.contains(",")) {
            validationErrors.add("name must not contain a ','");
        }
        if (name.contains("#")) {
            validationErrors.add("name must not contain a '#'");
        }
        if (name.contains("*")) {
            validationErrors.add("name must not contain a '*'");
        }
        if (name.startsWith("_")) {
            validationErrors.add("name must not start with '_'");