Re: When upgrading to a new version of tikiwiki, what changes to my site will be kept?
If you have edited template (.tpl) files, then they are likely to be overwritten when you upgrade your site. All of Tiki's interface text is made to be changeable. Normally the change is to present it in a different language, but you can use the same mechanism to customize the text for your site. You can make a file lang/en/custom.php and "translate" the text you want to modify.
See the file lang/fr/custom_example.php. Use this one as a base, just modify it and save it in the English language directory whose path I wrote above. Be sure to do the editing in a text editor that can save with utf-8 character encoding.
Using this method, your custom.php file won't be touched by site updates, and this will work for any of the interface text. To be sure what you want to customize is an interface text string, you could do a search in lang/en/language.php.
Update: I just noticed this is the old-fashioned way to do the customizing. Now you can also do this via the Tiki admin interface. Activate the multilingual feature, and under the "Customized String Translation" tab you can edit English language text.
-- Gary