It is currently not possible to change the author of a topic. So the user who is the first to save a new article as a draft or who publishes it will be (and stay) the author.

I agree that there are some use cases when being able to change the author would be helpful:

• User Generated Content: Registered Users can’t publish articles → you can only convert a discussion that was posted by a registered user into an article
• Company Content: If a company wants to publish all official content (like FAQ or product updates) from a company account instead of personal accounts → you have to share the login credentials of the company account with multiple people so they can publish content under the company name

Thanks, @bjoern_schulze . 

For any others who find this topic, please consider voting for this improvement idea: 

Our workaround to this is to have members create conversations (in a restricted category* so they aren’t public to the full community) and then convert them to articles in the moderator console.

*You’d have to give these authors a custom role so they can access the restricted category. 

We’re doing the same, @Kgastaldo!