Yes. Yes, by not wearing your retainers your teeth will move over time. Retainers are a life long function to maintain that where your teeth ended up after ortho will stay where they need to be. As you get older your teeth want to move even if you didnt have orthodontics. Since you have had ortho then your teeth will want to move even more.
YES! Your retainers were provided to you by your orthodontist to minimize changes in your teeth for years to come. Left alone, teeth will shift over years of use, whether you had braces or not. If they are tight, wear them more and take advil (ibuprofen) for the discomfort. Wear your retainers as long as you care about your teeth, bite and smile.