Aide-mémoire, should the need arise in the future to change hosting provider.
Copying the folder structure from legacy host to new host works fine, and avoids the need to find all the custom content that has been created over the years
Ensure the ftp client is set to binary mode, and that dotfiles and files with no extension are NOT treated as ASCII files (mainly relevant for .htaccess file and for image files, which corrupt if not transferred as binaries)
make sure that an imported/restored database is done forcing utf-8 character set (otherwise hieroglyphics appear)
before final database backup at legacy site, in Server Settings, set Force server URL settings to "No" (otherwise when restore database at new host, the phpbb login buttons force the domain name listed in server settings, and drive you back to the legacy board)
the root folder .htaccess file at the new host may or may not need amending. This is host dependent. Check the new host webserver log files to verify that cron urls are working ok (code 200 for success, rather than say, code 500 or 404 or some other error code)
autobackup does not fire when the board is disabled
strongly advise NOT to run the new board using a temporary domain name, while waiting for the actual domain to transfer. This would cause any board links created in posts to have the temp domain name hardcoded. When the actual .com domain is then migrated, those temp hardcoded links will fail.
remember that the logo and unread posts links on the landing page are hardcoded with the .com domain links, and will therefore take you to wherever the .com domain is (i.e. potentially the legacy site)
Some notes on the three methods of database import:
Using Import in the Admin Control Panel: can be tried, but will potentially fail due to either php timeouts or webserver timeouts. Larger database increases likelihood of a timeout.
Using phpMyAdmin in the hosting provider control panel: easy to use, takes care of utf-8 import, can still fail with timeouts though. Try and see.
Using linux command line at the console: needs ssh access to the webserver. Change directory to wherever the backup file is located. Command format is:
