Fat removed with liposuction is gone forever. You will always have fewer fat cells in the treated area. Because fat cells can grow with weight gain, for best long-term results, you should maintain a stable weight.
Note that liposuction does not correct loose skin. If the skin on your neck and chin stretches and loses resilience as you grow older, you may eventually benefit from a skin-tightening procedure, such as a lower facelift or a neck lift.