Just follow the steps below EXACTLY and everything will be fine. By making backups at each stage you can always revert to a working board if you encounter an issue.
Notes:
Your server must be running at least PHP 5.4.7 or higher.
phpBB 3.2 is not compatible with 3.0 and most of the previous files will need to be removed prior to upgrading.
Styles and MODs from 3.0 cannot be installed on phpBB 3.2. To upgrade, perform the following steps:
Make a back-up of the original files and database
Deactivate all styles except for prosilver
Remove all MOD-related changes from the database. The Support Toolkit's Database Cleaner can be used for this.
Ensure that the activated spambot countermeasure is one of the standard phpBB plugins (Simple image, GD image, GD 3D image, Q&A, reCaptacha).
Set British English as the only language pack and delete all other language pack directories from your server
Delete all of your phpBB 3.0.x files EXCEPT for the following:
The config.php file
The /images/ directory
The /files/ directory
The /store/ directory
Upload the contents of the phpBB3 directory from the uncompressed 3.2.x Full Package (EXCEPT for config.php) into your forum's directory (Note: you don't want to overwrite your original config.php file)
Browse to www.example.com/install/
Click the Update tab
Click the Update button
Select "Update database only" and click Submit
Wait for the progress bar to reach 100% and for a message indicating that the update has completed
That said, my experience is that the more posts you have the more likely you will get timeouts and issues using the web interface. Keep the CLI method in your back pocket and if you have 100,000 or more posts I would use CLI first.
In spite of attempts to make it bulletproof, I also often find that other database issues have to be fixed during an upgrade. For example, I often have to create the phpbb_config_text table manually. Don't be surprised if you get an error or two during any upgrade. You will just have to puzzle through it and make some correction, usually in phpMyAdmin or similar tool, to successfully upgrade. In addition, some steps seem like they will take forever, sometimes hours on larger boards. Just hang in there. They usually involve reformatting the phpbb_posts table.
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