In my new installation, about 100 users have signed up already. Most of them are still trust level 0.
I just realized that their topics will not be included in digests, which is bad – I want to use the digests to bootstrap the community. To remedy this, I set the default trust level to 1 (it’s okay – we don’t have realistic chances of spam posts).
How can I promote all my current users to trust level 1 if they’re still at 0?
You could manually promote everyone, or reduce the Tl1 requirements to allow Discourse to do the promotions automatically.
Oh thanks – I didn’t get the idea of dropping the requirements to zero 
I just set all tl1 requirements to 0, and will see what happens.
This should promote the users as soon as that are active on the site again, right?
I ended up using the API to do the promotion, since I was too impatient to wait for automatic promotion. Thanks!
Just do not set the default trust level to 1 unless you have extremely strict (paid) account vetting.
In my case, I’m sure there is no risk of spam – this installation is an internal forum for a course at university; it’s not even obvious that signing up will give you access to a forum.
But it’s good to put that warning out there: This eliminates almost all of Discourse’s spam-resistance.
Signing up for a university course sounds like paid vetting to me 