Bob:
I took your test case and "tweaked" it (modified a wprintf() and added eeh.ReplaceInModel()). I have included a .dgn file in the zip with a single text node. If you try and change the "B" in the last line of the text node to another character ("X" in this case) with a different font, it crashes. It will crash when you run the code in the zip file.
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Bruce