Will getting my first premolars removed before braces because of an overbite change my face negatively?

Positive change. This change will make your upper and lower teeth fit together more naturally. Ask your orthodontist to explain the process, and in this way you will be more comfortable with your treatment.
Evaluation. Orthodontic specialists are trained in facial balance for function, esthetics, and stability. Premolar extractions may lead to soft tissue drape changes, but the Orthodontist will take that into account. As you are concerned I would recommend that you do not proceed until you fully discuss your concerns with your Orthodontic specialist.
Difficult to predict. It can cause a narrowing and lengthening of the face I'm sure your case will turn out fine and you will be very happy with the results. See a 'functional orthodontist" for second opinion.