Is anyone aware of a way to obtain the freemarker source from a Transaction for PDF Layout to migrate it to a Advanced PDF/Html Template - assuming that source is transferrable (at least as a base to build upon)?

I’m not aware of any way of doing that within NetSuite, however, you don’t need that: There are multiple ways/tools to convert PDF to HTML, therefore the short path would be to generate the PDF and export/transform/convert that layout to html, then adapt the raw HTML to Advanced PDF template and you are done. One click conversion from regular template to Advanced PDF template is not a thing as far as I know.