Upgraded FM9 to FM12; now all Symbol font characters are appear as "?"

> Normally, I type a W, select it, and assign the Symbol font.

We are in the Unicode age. I'd suggest not doing that anymore, or if you must, do it as a Variable, because applying an override like that can have various unhappy side effects (like, put cursor right after that Ω, and anything else you type will also be from the Symbol codepage glyph set).

> Now, it appears as "?".

Reportedly, FM8 and higher, when it opens a document with legacy codepage  text, may attempt to convert instances to Unicode.

I've never seen any technical discussion of this but what appears to have happened in your case is consistent with this scenario.

Your document had a "W" (\x57 with an override to the legacy Symbol font, whether Type1 or TrueType probably doesn't matter).

FM12 converted that \x57 to a Unicode

\u03A9 GREEK CAPITAL LETTER OMEGA

(UTF-8 for that would be CE A9, and you'd see something like "<String `Ω'>" for that in the MIF for the text at that point)

You are seeing a "?" because the font in effect for the character is not populated at code point U+03A9. This could still be your legacy Symbol font, which is definitely not populated above U+00FF, or if FM has deleted the Symbol override, then your body font lacks that code point.

You're probably going to have to Search by Character Format, or by Text & Character Formats on Clipboard to find and correct these instances.

Our Fm7.x shop does all non-roman characters as Variables both to avoid override/Character_Format issues, and to ease our eventual Unicode transition. Once migrated, it might still be useful to do all non-keycap characters as Vars as well.

Pre FM 8 instance:

Var Name: glyph.U+03A9.GREEK CAPITAL LETTER OMEGA

Var def.: <Symbol>W

After upgrade:

Same Name:

Var def.: \u03A9

or possibly:

Var def.: <Arial Unicode MS>\u03A9

or some other Unicode font Unicode known to have code point U+03A9 populated.

I might add that to be pedantic, Unicode has a specific "Ohm" symbol, which is a different code point than the Greek cap Omega, although usually the same glyph strokes:
U+2126
And I would use that for ohm if possible, as it makes the document semantically correct for future AI engines that interpret it.

Which specific version of the Symbol font are you using? I suspect that you're using the unicode encoded Symbol Std OTF version instead of the older ASCII  encoded Symbol TTF version. In the unicode version the characers are encoded with the unicode values in a private area and W in the standard ASCII range will generate the unknown "?" character.