See http://learn.elgg.org/en/1.12/admin/duplicate-installation.html#database-entries (or maybe read the whole section). You can change site url, install folder (path), data folder (path) but then you need to update the corresponding entries in the database. I've linked the 1.12 docs as you seem to be using this version, right?

Do you want to keep only the users and get rid of all the content you users posted during all those years? If you just export/import the users you will lose ALL CONTENT (including messages and any uploaded files). Is this really what you want? If not, you need to upgrade your site from 1.8 to 3.0 step-by-step (1.8->1.9->1.10->1.11->1.12->2.3->3.0 - yes, you can upgrade from 1.12 directly to 2.3 but do not omit 1.9 - 1.12). You would also need to have compatible versions of any 3rd party plugin for the corresponding Elgg versions.

If all you're doing in your custom search hook is elgg_get_entities() why not simply register your subtype for search in the elgg-plugin.php (see http://learn.elgg.org/en/stable/guides/plugins.html#elgg-plugin-php)  and the system will handle the rest.

Thanks Jerome!

I registered my subtype in elgg-plugin.php of my plugin and it highlights only title and description of return entities not an owner of the entities when searching by username.

For example, Joe wrote blog posts and I wanted his name highlighted when searching for 'joe'. At the moment, it lists his posts without highlighting his name.