When upgrading MS products you should use always use a version that is equal or higher than the current version. Most of the time the installation program would check the installed version and refuse to install if "downgrading" is not supported, so you could try.
In all situations, the best option for not loosing data is making a full backup before the installation. It sounds obvious, but it's the best option.
In your case since it seems that there isn't a bundled package of Advanced Services SP1, I would uninstall the current version, install advanced services and then upgrade to SP1 (if possible always use the latest SP).
Other option is upgrading to Express 2008 R2, but note that R2 is a different version, so after upgrading you can't restore backups from 2008 R2 into another server that runs "just" 2008 (for some reason SQL Server never provided a method from restoring/importing a backup from a higher version).