Ah, I see, It's another property that was moved under config.
No, not just fork 😞 I thought, NodeBB changes will not rely so heavily on theme.
I have plans to build second version of the theme, that will be based on Persona v3
For now, my target is to fix most critical bugs (when something doesn't work at all)
The safest way is probably changing template use of username to first name and last name.
You could also hook into the user build event and change the output data that way, which would change it across the board, but I don't actually know what would happen... for the most part, actions are run against the user slug, so you'd probably be ok.