fl.font.file_name = "your new path"
fl.font.Save()
or
fl.font.Save("your new path")

A.

Yep, I already tried that. What happens is that FL saves a duplicate of my open font at "your new path." But the filename on the open font doesn't change (and thus if I hit "save" again, any changes go into the old file.)

How about:
fl.font.file_name = "your new path"
fl.UpdateFont(-1)
fl.font.Save(fl.font.file_name)
?

That works. Thanks.

Does a fontindex of -1 have a predictable result in other functions that ask for a fontindex, or is this a special behavior of UpdateFont()?

Can also be done through Robofab

new_file_path = [whatever you want]
this_font.save(new_file_path)
this_font.update()

fl.UpdateFont(-1) is synonymous with fl.UpdateFont(fl.ifont) in this case, but it's not universal behavior, I think. Actually, using fl.UpdateFont(fl.ifont) is more elegant to use than fl.UpdateFont(-1).

Geez. Amusingly, what I *need* is the behavior Matthew B was trying to avoid. Here I was going through some extra contortions to get this behavior. Re-reading this just enabled me to drop a bunch of lines of code as unnecessary. :)