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I just did the same thing myself - moved XOOPS from a subfolder to root.
I edited mainfile to reflect the new settings and everything worked perfectly.
I'm actually quite impressed about how easy it is to move XOOPs to another folder or even a new server - kudos to the design team!
However, you may have to update some absolute URLS that you have entered yourself, to reflect the new site location.
If you have to update any absolute URLs in the DB, try this handy MySQL function:
update [table_name] set [field_name] = replace([field_name],'[string_to_find]','[string_to_replace]');
I ran it from PHPMyAdmin as SQL against some tables (articles - article_text and wfdownloads - URL) to update the absolute URLs and it worked like a charm
Like any database operation, always backup your table/data first (PHPMyAdmin - Export>SQL).
It certainly saved time rather than picking throught the front-end text.