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. 

{
    
    ReplicatedMergeTreeQueue::SelectedEntryPtr selected;
    try
    {
        selected = queue.selectEntryToProcess(merger_mutator, *this);
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
    }
    return selected;
}
bool StorageReplicatedMergeTree::processQueueEntry(ReplicatedMergeTreeQueue::SelectedEntryPtr selected_entry)