Thanks for clarifying, Lumpy.
I found that if I don't change the table prefixes, phpBB ignores my imported data and it remains a fresh, empty board (no users, groups, posts, etc. showed up). And when I kept the table prefixes the same, phpMyAdmin complained on import that the first table already exists and it failed to import anything.
I did spot config.php and tried to edit the table prefixes there. But it broke the forum, meaning nothing came up in my browser except an empty white page. Quite literally empty: viewing the page's source code revealed only the opening and closing html, head, and body HTML tags with absolutely nothing else.
I just tried it a fourth time. Same failure: I get the fresh install to work and get the welcome page, and after importing the database via phpMyAdmin and return to the install folder, I just get a blank page. I kept the table prefixes different this time (phpbb3_ for the old tables, phpbb_ for the new). I modified the config.php file to use the phpbb3_ prefix before going to the install page. When I change it back to phpbb_, the site comes up again, but it's empty of posts, users, and other data. The log tables (phpbb3_log and phpbb_log) show no errors or other information related to this.
And thanks janus, will do. I'm not seeing any errors to report. Just blank pages when it fails.