Right-on with "replace files and update DB", that seems like the sanest way to go.
Just putting this here for consideration - the phpBB 3.2 EOM and Ubuntu 20.04 LTS release date are within weeks of each other. One could, assuming an Ubuntu phpBB install:
- Wait until at least April 23rd, when Ubuntu 20.04 LTS goes live. More conservative folk will add an extra 2 weeks.
- Put forum into maintenance. It'll be down for a while, possibly hours.
- Snapshot the instance. Just in case things go horribly, terribly wrong and you need to be back to where you were. Also, for "method 2" with minimal downtime.
- Optional method 2: Spin up a new instance from snapshot. Do all upgrade work there. Bring forum back out of maintenance. Profitably, you may want to set DNS TTL to 1 minute, one day before all this maintenance.
- zip or tar forum files; dump the DB into a file.
- Upgrade the base OS and applications. This will break a phpBB 3.2 forum because PHP. Use this as a house cleaning exercise - I at least intend to. PHP to 7.4 or 7.3, depending on which ultimately ships with 20.04 (I assume 7.4); if still on MySQL 5.7, consider MariaDB or PostgreSQL; give apache2 and let's encrypt / acme a good scrubbing.
- If DB software changed, you may need to bring the dump in - check to see the DB works and can be connected to via username/pw used for phpBB
- And upgrade to phpBB 3.3.x with "fresh files / update DB" method. config.php might need a touch depending on how much changed for the DB.
- Come out of forum maintenance, test
- Optional method 2: Forum back into maintenance, one last DB dump, bring that into the new 3.3 instance, upgrade DB again, swing DNS over, power down 3.2 instance. Watch for a day or so, if everything is fine (or good enough), kill 3.2 instance to save hosting $.
Of course it's also possible to stick to 18.04/mySQL 5.7/PHP 7.2. 18.04 will still be supported when 22.04 rolls around. Personally, I like being on the "current" LTS - 2 years between OS upgrades is fine, I don't need it to be 4 years. Your preferences may be quite different.