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
Metadata.builder(stateWithTemplate.metadata())
                    .put(IndexMetadata.builder(temporaryIndexName).settings(finalResolvedSettings))
                    .build()
            )
            .build();
        final IndexMetadata tmpIndexMetadata = stateWithIndex.metadata().index(temporaryIndexName);
        indicesService.withTempIndexService(tmpIndexMetadata, tempIndexService -> {
            
            MetadataCreateIndexService.resolveAndValidateAliases(
                temporaryIndexName,
                Collections.emptySet(),
                MetadataIndexTemplateService.resolveAliases(stateWithIndex.metadata(), templateName),
                stateWithIndex.metadata(),
                
                
                xContentRegistry,
                tempIndexService.newSearchExecutionContext(0, 0, null, () -> 0L, null, emptyMap()),
                IndexService.dateMathExpressionResolverAt(System.currentTimeMillis()),
                systemIndices::isSystemName
            );
            
            String indexName = DataStream.BACKING_INDEX_PREFIX + temporaryIndexName;
            
            List<CompressedXContent> mappings = collectMappings(stateWithIndex, templateName, indexName);
            try {
                MapperService mapperService = tempIndexService.mapperService();
                for (CompressedXContent mapping : mappings) {
                    mapperService.merge(MapperService.SINGLE_MAPPING_NAME, mapping, MapperService.MergeReason.INDEX_TEMPLATE);
                }
                if (template.getDataStreamTemplate() != null) {