When I upgraded my test system I had the luxury of setting my PHP timeout to -1 as per the CLI but I will not be able to do this to the live system as my hosting service for obvious reasons will not allow it.
The release notes suggest doing this
2 questions 1) Is my board root /phpBB and 2) can this be run once I am there (in the board's root) by SSH and how is the progress reported?From your board's root, execute the following command: php ./bin/phpbbcli.php db:migrate --safe-mode