If they still fit, it should be possible, if you are not too far away. Now to achieve result, you will have to wear the appliance all the time exept to eat and clean your teeth. You are now back into active treatment and must act as such. When the result would be achieve, keep the active treatment of another 6 months and have your orthodontist make you new retainers. It is more than recommended to have your orthdontist monitoring the whole procedure because he/she is the only one to be able to confirm to you if the retainer alone can correct the problem. This answer is purely therorical and physical examination of your mouth is required.