This PDF explains how it was supposed to work at the FM7.x to FM8 transition:
It looks as if it only works optimally on the same system configuration (esp. operating system and Fonts installed) that created the older documents. I do not know if FM9 and later act similarly. I have no experience with font migration in Frame, as we are still on 7.1/Unix.
I see traffic on this forum suggesting that Marker Text does not get converted. Here's a thread on that:
Here are some other related discussions:
A further issue with the transition from overlay/codepage fonts to Unicode is whether or not the desired typefaces are available as Unicode, and whether or not they have all the required glyphs populated at the Unicode code points used in your documents.
Assuming that any arbitrary "Unicode" font covers more than a fraction of Unicode codepoints would be a mistake. Generating, scaling, hinting and otherwise optimizing 110 000 glyphs is a daunting task. I wouldn't be surprised if no single font has covered it all yet (other than Adobe Blank). The question then becomes: what's missing? I suspect most "Unicode" fonts have only a tiny fraction of Unicode space covered. There's also the question of whether variations (italic, bold, etc.) match the basic coverage.
You might expect font foundries to provide pages or PDFs detailing all the Unicode scripts a font covers, by variation, and showing at least a bitmapped preview of what each glyph looks like. This doesn't seem to be the case.
A few minutes in Adobe's Type Products discovered no obvious means of determining codepoint coverage. A couple of other big name foundries at least had icon sets, each icon representing a Unicode Script set, but no comprehensive list of codepoints covered. At least one site allowed pasting/typing arbitrary text into a dialog so you could see if a particular glyph was populated, and what it looked like. Hope you don't have questions about too many.
Unless I've missed something, this situation appears to be a mess.
And how does FM8+ behave when the document contains a codepoint not implemented by the font applied?
 or [?]?
Switches to font that has the codepoint?