Do you formally interview for internal promotions? What do I wear if I'm interviewing for Lead or Store Manager?

I left the company from being a Sales Associate and applied for an Ops Lead position. I am expecting an interview since I'm returning. May be silly but my first question is what do I wear? I have no idea what's too fancy for this environment.

I'm hoping to move up the ladder and become SM and am interested what the process looks like. Any advice is much appreciated :)