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. 

{
    ReplicatedMergeTreeQuorumAddedParts::PartitionIdToMaxBlock max_added_blocks;
    for (const auto & data_part : getDataPartsForInternalUsage())
    {
        max_added_blocks[data_part->info.partition_id]
            = std::max(max_added_blocks[data_part->info.partition_id], data_part->info.max_block);
    }
    auto zookeeper = getZooKeeper();
    const String quorum_status_path = fs::path(zookeeper_path) / "quorum" / "status";
    String value;
    Coordination::Stat stat;
    if (zookeeper->tryGet(quorum_status_path, value, &stat))
    {
        ReplicatedMergeTreeQuorumEntry quorum_entry;
        quorum_entry.fromString(value);
        auto part_info = MergeTreePartInfo::fromPartName(quorum_entry.part_name, format_version);
        max_added_blocks[part_info.partition_id] = part_info.max_block - 1;
    }
    String added_parts_str;
    if (zookeeper->tryGet(fs::path(zookeeper_path) / "quorum" / "last_part", added_parts_str))
    {