I've searched the site and have seen this brought up since 2011. However, none of the answers apply.
I am testing Jira v8.0.0 and while I am able to change the Project Lead, there doesn't appear to be any way to change the Default Assignee. The screen shot below indicates I should be able to update it, but it cannot be edited.
Is there a secret sign-in / handshake / bribe needed that I don't know?
