As long as there are no changes in the anatomy of the teeth, such as  tooth movement, a filling, a crown, lost or broken teeth, broken or missing filling, we can use the original impressions to make aligners. If there is an anatomical change, then the you must make a new set of impressions ($49 fee). If you have worn braces in the past, but stopped wearing a retainer, it is likely that your teeth have been or are moving. Therefore we advise that you send in your impressions as soon as you make them, and that you order your Aligner Treatment as soon as you receive your Treatment Plan. This is important because we want - and you want- your aligners to fit perfectly. Perfect aligners require perfect impressions. :-)