Because you are downgrading the version of Roundcube I do not think you can simply copy the config from one to another. I'd reccomend that you go through the settings manually to ensure there are no issues.
<rc root>/config/config.inc.php is the default location of the Roundcube config file. You could check if when your version of 1.2.3 was setup it was done wrong and who it was put your config into <rc root>/config/defaults.inc.php rather than config.inc.php other than that there is no really no way for anyone to guess how your server is configued.