I'd like to share my journey upgrading from 6.2.3 to 7.7.4. I hope this may help anybody avoid hiccups.

Original Version: 6.2.3
First upgrade to: 6.5.5
Then install and import to: 7.5.6
Then upgrade to: 7.7.4

I spun up two local hosts on a Windows 10 desktop. I use UniserverZ as my primary but was forced to grab an old version of xampp due to mysql incompatibility.

Your setup may differ. You may find it easier to start off with an older php/mysql config and upgrade them while you install each new version of ubbthreads.

This post will follow the steps that I took - starting with uniserverz and occasionally switching over to xampp.

1: UniserverZ XIII with both PHP 5.4 and PHP 7.3, and mariadb 10.3.17.
2: XAMPP v1.5.2 with PHP 5.2 and mysql 5.0.x.

Have UBBThreads 6.2.3 and your database set up on UniserverZ pointing to PHP 5.4.
Start UniserverZ (leave xampp alone for now).
For the purposes of these instructions, ubbthreads is set up at: c:\UniserverZ\www\ubbthreads\ .

Go to mysql config.
Add this value:
See this thread for more information.

Back up database and code.
Store your database:
mysqldump --user=user --password=password database > ubbthreads623Backup.sql 
Zip your current codebase.

Install UBBThreads 6.5.5
Remove all UBBThreads 6.2.3 files.
Upload the UBBThreads 6.5.5 files to c:\uniserverz\www\ubbthreads\ .
Using your backup, copy these files from the old instance to the new instance:
* config.inc.php
* main.inc.php
* theme.inc.php
Paste those three files to ubbthreads/includes/

Upgrading database from 6.2.3 to 6.4.1
Follow the steps located here: http://localhost/ubbthreads/UPGRADING.html
In summary, run the following scripts:
The last statement in each script should be "Done".
If your browser times out, hit Refresh until you see Done.

Upgrading database from 6.4.1 TO 6.5.5
Navigate to http://localhost/ubbthreads/install/
Follow the instructions onscreen.

Copy themes from 6.2.3 to 6.5.5
This step is optional since you likely will be discarding themes on your quest to 7.7.4+.
Copy any special theme files from your original ubbthreads/stylesheets to ubbthreads/stylesheets.

Test: Launch UBBThreads and log in.
Test should be successful.

Back up 6.5.5
Take backup of database:
mysqldump --user=username --password=password databasename > ubbthreads655backup.sql 

Switch instances.
Kill UniserverZ.
Launch xampp and load apache/mysql.
Launch phpmyadmin.
Create empty database1 (this will hold your 6.5.5 data).
Create empty database2 (this will hold your 7.5.6 data).
Create a user and grant permissions to both databases. You may use a different user for each database if preferred.
Copy your database backup file ubbthreads655backup.sql to xampp.
Import the data to xampp. Note that newer mysql commands do not work on this version of mysql. Launch a command line window:
    cd c:\xampp\xampp\mysql\bin\
    mysql.exe --user=root database1
    mysql> SOURCE c:\xampp\xampp\ubbthreads655backup.sql 

Installing 7.5.6
Background: 7.5.6 is a known stable version that works with the UBBThreads 6.5.5 Importer.
Here, we will install a clean instance against an empty database, and then migrate the old data to the new database.

Copy UBBThreads 7.5.6 files to: c:\xampp\xampp\htdocs\ubbthreads\
Launch the installer at http://localhost/ubbthreads/install .
Follow steps onscreen to create empty tables.

Migrating Data from 6.5.5 to 7.5.6
Important note: The import will take a long time to run. Depending on the size of your forum, it may be hours or worth doing overnight.

Download ubbthreads_6_5_import under the Members Portal.
Copy threads_import.php to ubbthreads/importers/
Launch http://localhost/ubbthreads/importers/threads_import.php .
Follow the steps. Specify 'localhost' in the first field.

Test: Launch UBBThreads and log in.
Test should be successful.

Back up 7.5.6.
Note: I could not figure out the command to dump the database. smile
Launch http://localhost/phpmyadmin and export the database to a file: ubbthreads756Backup.sql.

Switch instances.
Kill xampp.
Launch UniserverZ.
Load apache/mariadb. Use the PHP 7.3 config (or any 7+).
Launch phpmyadmin and create a new empty database2 (this will hold your 7.5.6 data).
Upload the data:
mysql --user=username --password=password database2 < ubbthreads756Backup.sql. 

Installing 7.7.4
Delete the existing ubbthreads 6.5.5 directory.
Copy UBBThreads 7.7.4 files to the ubbthreads directory.
Launch: http://localhost/ubbthreads/install/upgrade.php
Log in as administrator and follow the steps.

You may encounter an error: The following files do not exist in your install.
If this occurs, navigate to /install/upgrade_changes.txt, remove the offending lines and rerun upgrade.php.
Background information and examples.

Test: Launch UBBThreads and log in.
Test should be successful.

Fix forum settings.
If images are not correctly appearing, you will need to edit the forum styles.
Navigate to Control Panel -> Display -> Styles.
Click on a Style.
Under "Name, Images & Wrapper", ensure each is set correctly and then click Update.
Repeat for each Style.
Background and more information.

Forums may not display correctly for all users.
Navigate to Control Panel -> Permissions -> Forum. Scrutinize every setting and ensure they are correctly set.

Back up 7.7.4.
Take backup of database.
Zip the UBBThreads codebase.

At this point the database and files should be ready to move to your production server.