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
.put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, shardReplicas)
                .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
                .build();
            final IndexMetadata tmpIndexMetadata = IndexMetadata.builder(temporaryIndexName).settings(dummySettings).build();
            IndexService dummyIndexService = indicesService.createIndex(tmpIndexMetadata, Collections.emptyList(), false);
            createdIndex = dummyIndexService.index();
            if (mappings != null) {
                dummyIndexService.mapperService().merge(MapperService.SINGLE_MAPPING_NAME, mappings, MergeReason.MAPPING_UPDATE);
            }
        } finally {
            if (createdIndex != null) {
                indicesService.removeIndex(createdIndex, NO_LONGER_ASSIGNED, " created for parsing template mapping");
            }
        }
    }
    private void validate(String name, ComponentTemplate template) {
        validate(name, template.template(), Collections.emptyList());
    }
    private void validate(String name, ComposableIndexTemplate template) {
        validate(name, template.template(), template.indexPatterns());
    }
    private void validate(String name, Template template, List<String> indexPatterns) {
        Optional<Template> maybeTemplate = Optional.ofNullable(template);
        validate(
            name,
            maybeTemplate.map(Template::settings).orElse(Settings.EMPTY),
            indexPatterns,
            maybeTemplate.map(Template::aliases).orElse(emptyMap()).values().stream().map(MetadataIndexTemplateService::toAlias).toList()
        );
    }