Will orthodontists use original moldings or new moldings for new retainers?

Probably new ones: If it hasn't been too long, and your orthodontist still your old models, they may be able to use them. But since teeth move, it's more likely that they'll take new impressions and make new models from them.

New models needed.: You will need to have new impressions taken from which new models will be made and the retainers from the new models. Most ortho offices do not keep old models.

Braces: New molds are needed because if your teeth have shifted even a little, making a new retainer on the old molds will result in a new retainer that won't fit.

New: retainers need to be made to fit where your teeth are now, not where they were at some earlier point in time, so new molds will be required

Probably not: By original mouldings, i'm assuming you mean impressions of your teeth at the beginning or prior to orthodontic treatment. And if that is what you mean, & you've elected to go through with either traditional braces or clear aligner therapy, then at the end of treatment, your teeth will be drastically different than they were at the beginning of treatment. New impressions will be taken at the end.

Not likely: The mouth is always changing...So if you are getting new retainers they will most likely take new impressions. Your original impression are probably old from before treatment was started and now your mouth is not even close to you new smile.

Probably not: The fabrication of retainers is a destructive process to the model. Most do not survive the process. You will likely need to take new impressions.

A rule of thumb: New molds are always required for new retainers.