What should I wear?

I already do the job part time, but this will make it fulltime for a year. They know me/ respect my work/ laugh at my jokes. I'm not the only candidate and don't know who else has applied.

Do I need to fully suit up for the interview or can I get away with adding a tie to my regular office outfit? Do I need to worry about first impressions if I already see most of the panel regularly anyway?