<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
    <connectionStrings>
      <add name="DefaultConnection"
        connectionString="MinhaConnectionStringDeProdução"
        xdt:Transform="SetAttributes" xdt:Locator="Match(name)"/>
    </connectionStrings>
</configuration>

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
    <connectionStrings>
      <add name="DefaultConnection"
        connectionString="MinhaConnectionStringDeDebug"
        xdt:Transform="SetAttributes" xdt:Locator="Match(name)"/>
    </connectionStrings>
</configuration>