New retainer with current state of teeth will maintain them
It is possible that your previous retainer will help move the teeth back closer to the condition they were in when you finished your treatment. This is going to depend on how much they have moved, how your teeth started off, how the retainer fits, and the type of retainer. Being able to make a retainer from it will also depend on whether there was any damage to the model as your original retainer was made. It is probably a good idea to suggest this to your doctor to see if it is possible. Good luck!