Will orthodontists use original moldings for new retainers?

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.