Do I have to wait for UddeIM 2.9?

There are two ways:

1. Copy \administrator\components\com_uddeim\config.class.php
(contains the active configuration)

2. Use uddeIM's internal backup (maintenance tab). Since you copy all database tables you should be able to restore the config.

Perfect, thanks for your quick reply! To be clear, no table structure changes between 2.8 and 3.1?

Yes, there were no table changes.