Because your teeth have already shifted, they may do more shifting over time. The orthodontist doesn't just make retainers for the fun of it, they are meant to retain your teeth in the final position. The longer you wear your retainers, the better ability your teeth will have in retaining the position on their own. Teeth have a "memory" of their original positions. It also depends on HOW your orthodontist straightened your teeth there are different ways of doing orthodontics. Also your orthodontist should have made and kept a model of your teeth on your last/final orthodontic visit. This is so that he can re-make any retainers if he needs to. But you will have to pay the costs of replacing them. If your teeth have shifted to the point that the old retainers won't fit, then you will probably have to do more ortho to correct the problem, maybe just a series of retainers to re-position them. You can also ask your orthodonitist to place a perment retainer. A thin wire along the inside of your teeth that will hold them in place. And finally if you need to re-do ortho you may want to consider the new invasalign system.
Good Luck
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Will your teeth move a lot if you don't wear your retainer? 
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