Hi
I want to install a 1.8 version and transfer all required database tables from 1.6.14 to the new forum. Can such an update be done or is there any changes in the 1.8 database?
I suggest you duplicate your 1.6 forum and upgrade it as a test before doing the same for your production forum. That will give you time to make necessary modifications since the themes and plugins need to be modified or changed.
If you have the server resources, duplicate forum by copying existing database to new one with different name, then creating a new directory /forum2 and copying everything from /forum, then editing the /forum2/inc/config.php to match new location. Delete /forum2/inc/settings.php, then use phpmyadmin to modify records in mybb_settings table. Change bburl to point to new location, and I suggest you change boardclosed value to 1. To differentiate the test forum from live, you should also change bbname record to TestForum or something similar. That will show in the navigation breadcrumbs.
Log in to the duplicate forum and verify it is operating as you expect it to. As admin, confirm configuration settings. As forum member, create a test post, and do all the normal forum activity to verify operation is correct. Monitor the browser URL to verify everything stays in /forum2
Once you have cloned your forum, then do the upgrade in /forum2. Otherwise you run the risk of interrupting normal activity by your userbase.