The first topic above is the way it has been suggested by the Opencart developers to upgrade in case the core upgrade would failed.
However, followed is my version on how I did proceed with the upgrade with the OC v126.96.36.199 release on a test server
when failed to upgrade:
1 - Backup your previous database of Opencart version from your PHPMyAdmin
2 - Ensure to have created a new database table for Opencart v188.8.131.52 in your PHPMyAdmin
3 - From your SQL file exported from your previous version of Opencart, open it. Ensure that all URL locations points to the right locations for the new version.
4 - Copy your root config.php file and admin/config.php file and ensure to make the modifications for both database which should point to your new database name. Also ensure both files contains the right paths.
5 - From the opencart v184.108.40.206/install folder, copy the oc_cart and oc_banner_image table aside (CREATE and INSERT query - both).
6 - Either upload or paste the entire SQL backup content into your SQL query tab on the new Opencart database from your PHPMyAdmin and execute it.
7- Paste the aside copy from step 5 into the SQL query tab on the new Opencart database from your PHPMyAdmin and execute it.
8 - From your SQL backup file, it may be possible that you might need to copy and paste the user_group table query as well as the insert if the new database provides only the demonstration or empty results from that table on your new Opencart database. If so, simply copy both the CREATE and INSERT statements and paste them into the SQL tab on your PHPMyAdmin on your new Opencart database. Then, execute it.
9 - Log in to your OC admin. If unable to login, ensure to check your user database table from your new database to see if the administrative account is still there and active. If it is and even if it isn't and you're unable to login, copy your INSERT statement block from your user table from your SQL backup file and paste it into the SQL tab from your PHPMyAdmin. Then, execute it and refresh the admin page (twice). Then, try to login again.
10 - Go to your admin - > systems - > users - > user groups and ensure all pages are unchecked and checked where the most administrative rights applies.
11 - New ways of handling extensions
: The admin extensions are now handled into a sub-folder for the 4 categories of the folders - controllers, language and view. Ensure that all extensions are compatible as mentioned on my first post with their relative paths: e.g: under the controller/extension/extension/<your_extension>.php
12 - The store-front end's extensions are now handled in e.g: controller/extension/<your_extension_folder>/<extension_file_name>.php
13 - In your admin - > systems - > settings - > edit store, ensure all settings (and new settings) are properly configured.
14 - In your admin - > extensions - > extensions page, use the dropdown menu. Ensure all relative extensions are active.
If this step does not work as it should by providing error messages from the screen, for instance, please read my first post regarding downloaded extensions with the latest release of Opencart. If not compatible and you're unable to remove them, use the 'search' tab feature in the PHPMyAdmin and type in every single filenames that the error message will provide you (just the last portion of the path). Then, delete the found entries accordingly and refresh your site.
15 - Once all the above have been configured, load your store-front end.
Note: Step 14 is of course a temporary error message until each extensions developers wlll fix the issues on their end. In addition, more information can be found regarding how to handle extensions location in Opencart v2.3 stable release: http://forum.opencart.com/viewtopic.php?f=183&t=165958
Followed is a screenshot based on the succession of the above operations.