@Per Christian Strand - When someone is shown as having a Free License in a paid plain, e.g., Team Plan, this is because someone added/invited them at the board-level (usually as a viewer, but commenter may also work this way too).

Once this is done, someone can make them a Full License user, but not the other way around. You will need to delete or remove them from the team.

If they need to be edit boards that reside in a paid team, they either need to be a paid user or  you will need to use the guest editor feature.

By the way, here is a recent post where I demoed how someone is addrd as a Free License user:

https://community.miro.com/ask-the-community-45/non-admin-joining-peple-to-the-team-4604?postid=19242#post19242