The configuration data and state information for each individual Reminder web part is stored in the web part properties which is then stored by SharePoint in the content database - so whatever backup/upgrade method you choose for SharePoint itself will also cover Reminder.

Some configuration information (e.g. the license key) used by the Reminder service is stored in the programs installation directory (usually "c:\program files\pentalogic\sharepoint reminder\Pentalogic.SharePointReminder.Service12.exe.config" and the services "Log On User" is set in the services control panel (services.msc).

You will need a new License Key for version 4.x which is free of charge if you have an active Premium Support agreement.

Please install Reminder in the 30 day trial mode and send us your new Installation Code along with your purchase details and we will return a new license key. See the Licensing FAQ.

Database Attach Upgrade

See Microsofts official guidance on SharePoint Server Subscription Edition upgrades for SharePoint itself.

On the old server ensure you stop the SharePoint Reminder service and/or remove the program via Add/Remove control panel to ensure you don't inadvertently get email alerts from the old server (your settings will be saved)
Download and install the latest version
Some configuration information (e.g. the license key) used by the Reminder service is stored in the programs installation directory (usually "c:\program files\pentalogic\sharepoint reminder\Pentalogic.SharePointReminder.Service12.exe.config" so either copy this file over or update the settings manually
Send us your new Installation Code along with your purchase details and we will return a new license key. Licensing FAQ
Follow the additional setup instructions.
Run the Diagnostics tool all the way to the test email at the end to ensure everything is working correctly