Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

{
    String full_zk_path = getZooKeeperName() + getZooKeeperPath();
    if (!restorer.getRestoreCoordination()->acquireInsertingDataIntoReplicatedTable(full_zk_path))
    {
        
        
        return;
    }
    if (!restorer.isNonEmptyTableAllowed())
    {
        bool empty = !getTotalActiveSizeInBytes();
        if (empty)
        {
            
            
            StorageReplicatedMergeTree::Status status;
            getStatus(status,  false);
            if (status.queue.inserts_in_queue)
                empty = false;
        }
        auto backup = restorer.getBackup();
        if (!empty && backup->hasFiles(data_path_in_backup))
            restorer.throwTableIsNotEmpty(getStorageID());
    }
    restorePartsFromBackup(restorer, data_path_in_backup, partitions);
}
void StorageReplicatedMergeTree::attachRestoredParts(MutableDataPartsVector && parts)