Will I "undo" my lower face lift if I lose weight after the surgery? Should I try to lose the excess weight before?

If you are going to be losing weight, I would recommend doing this before having your lower facelift.  If you lose the weight after the surgery, it could affect your results.  

If you are planning on losing a considerable amount of weight, I would recommend getting a start on that before surgery.  There are some areas that may not change even if you do weight (areas underneath the chin that may have more to do with your genetic predisposition), but you can have extra skin laxity in the face.  If you are close to your weight loss goals and only have a few pounds to go, you would not "undo" your surgical results.  Be sure to discuss this with your surgeon at your consultation so you can be advised appropriately. Best of Luck