Replacing the default font used by Framemaker with a specific proprietary font

It sounds like you would like to update existing company documents with new branding requirements, and create new docs that follow those standards as well. Is that correct?

If so, it's helpful to know that unstructured FrameMaker templates aren't physically connected to your documents like they are in structured documents, or in other unstructured applications. 

You'll want to update the stored definitions in your template(s) with the Paragraph Designer and Character Designer as needed as you need, and then use File>Import>Formats to  bring in the updated styles from your template file into the content documents.

-Matt

Another thing to watch out for on this template/default transition is that proprietary font (PxC GillAltOneMT, V2.000). How much of Unicode BMP space does it populate, and does it cover all the code points your documents require?

After the font change, be sure to check a variety of output workflows, looking for unexpected instances of question mark characters [?]. FM does not have font fallback, nor does it throw any dialogs when a font in use doesn't have a character called for by the text.

Do you mean the font in FM dialogs,
or the font in the factory-supplied templates?

The templates can be revised (with suitable filesystem permissions).
What FM uses internally I tend to doubt (if only because the new font might need to populate the entirety of Unicode BMP space).