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. 

{
    LOG_INFO(log, "Starting up tables.");
    
    const size_t total_tables = TSA_SUPPRESS_WARNING_FOR_READ(tables).size();
    if (!total_tables)
        return;
    AtomicStopwatch watch;
    std::atomic<size_t> tables_processed{0};
    auto startup_one_table = [&](const StoragePtr & table)
    {
        table->startup();
        logAboutProgress(log, ++tables_processed, total_tables, watch);
    };
    try
    {
        for (const auto & table : TSA_SUPPRESS_WARNING_FOR_READ(tables))
            thread_pool.scheduleOrThrowOnError([&]() { startup_one_table(table.second); });
    }
    catch (...)
    {
        
        thread_pool.wait();
        throw;
    }
    thread_pool.wait();
}
void DatabaseOrdinary::alterTable(ContextPtr local_context, const StorageID & table_id, const StorageInMemoryMetadata & metadata)