Why would they make new molds if the teeth have shifted already? will they adjust the retainer somehow to put the teeth back into place? or will they

Teeth shifted: The reason for taking new moulds is to register the current position of the teeth. This will allow the dentist to create a retainer that fits your teeth now. The fitted retainer can then be adjusted to move your teeth to a new position.

Unknown: It is hard to say why new impressions are needed when we have no idea whether you are currently being treated orthodontically or if you have relapse. The best person to answer your question is the dentist who is treating you.

Shifted teeth: Retainers are not tooth moving appliances, they are tooth holding appliances. If Dentist or Orthodontist feels there has been substantial relapse, old retainers may no longer be adjustable. New model to fabricate new retainer may be required to prevent additional shifting. To try to recapture what has been lost requires more than a static appliance.