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

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.
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. 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.
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.