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I understand you are careful and don't want to mess up.
But it is possible to make a complete back-up of
- the site (html and php-files)
- the database
and it is fairly simple. That way you can always revert to the old 2.0.3 site of yours if the upgrade somehow messes up.
The site can simply be back-upped by copying everything on your site to a dir on your harddisk.
The database can be back-upped by going into the MySQL administration provided by your host. But I admit, that can be a pretty scary experience. There is a module (simply called "Backup"), which places a button on your homepage. When you press it, it makes a backup of your database and sends you an email telling you were to download the backup, complete with a link. Very easy to use.
You should consider to make backups of your site at regular intervals as your host provider is bound to screw up sooner or later.
We are now in the process of updating two sites from 2.0.3 to 2.0.6 which is somewhat complicated, as there a multitude of hacks which we have to move along. Until we have finished that successfully, I can't say I am experienced at updating XOOPS