I replaced the glyph for U+3054 of a ttf font (mplus-1m-light from http://mplus-fonts.sourceforge.jp) with that from a different font for the same encoding (mplus-2m-light). I exported the file with autohinting turned off (since I’m interested in using the monospaced kanji). The new .otf shows the correct/replaced glyph for that encoding.
However, when I then tested the file as a webfont, I found that a handful (possibly many more) of glyphs were missing in the exported version.
Here is the mplus-1m-light font used as a web font on a page designed to test if it supports certain Japanese characters for radicals: http://kanjialive.com/radical-test.html You’ll see that in the first list, all necessary glyphs are present (the characters were all encoded as shown).
Now here is the same page again, this time using the edited, .otf version of the font exported from Glyphs Mini http://kanjialive.com/radical-test2.html. You’ll see that some glyphs are missing and were rendered using the fallback fonts.
The glyphs I see as missing (and which I can’t seem to select in Glyphs Mini either by their Unicode entry) are:
I will have a look at this but it will take a while. Meanwhile, please note that while you can import existing compiled fonts by opening them, Glyphs cannot reverse-engineer all information compiled in it. So after export, you will always have a different font. Please read this: http://glyphsapp.com/blog/importing-existing-fonts/
Glyphs Mini does not properly support CJK fonts. And the glyph info definition for the “CJK Unified Ideographs Extension B” is not jet part of the glyph data base. Thats why they are not correctly encoded in the final font.
The three glyphs use the second upper plane and have five digits unicodes…
For the glyphs to work properly, you need to change the name to the Unicode convention, i.e., g7146 ought to be called u201A2, etc.
For the task you want to accomplish (simply copying glyphs from one file into another), I would rather use DTL OTMaster, or, if you are good at the command line, you can use tx or mergeFonts from the AFDKO. These tools preserve more of the original information in the compiled file.