re: if I want to convert back a FM file into a MIF, is it sufficient to rename the file extension or must be done through the "Save As" function of FM.
If the file actually was a binary .fm file to begin with, simply changing the filename extension does nothing but change the filename extension (which might affect what app Windows dispatches by default, but does not convert the file contents to MIF). To get a plaintext MIF file you must Save-As MIF. Older FM's, by the way, didn't even automatically offer a .mif extension.
There probably are tools and scripts to automate the .fm to .mif process; Mif2Go, perhaps.
Internally, a .fm binary file begins with a single line of plaintext, like: <MakerFile 7.0H> followed by pure binary content incomprehensible to a plaintext editor (and to most apps other than FM itself).
Internally, a MIF file is entirely plaintext, and begins with (for example):
<MIFFile 7.00> # Generated by FrameMaker 7.0p578 followed by plaintext <> markup and #comments, which can be parsed easily (I've written scripts to do so for goals unrelated to the current topic).
When opening a single file, I suspect Frame doesn't really care about the filename extension, and may rely on the initial data in the file (this would be expected for an app that got its start on Unix, where filename extensions were not typical practice, but Magic Numbers were - the first 4 bytes of FM and MIF are unique and may well have been the magic numbers).
The extension does matter, of course, for books, where the component files are expected to have ".fm" name format.