In case it’s of any use to anyone, I’m posting below a way to modify the Change Rich Text Font script so that it preserves bold, italics, and bold-italics. (The example uses Helvetica 13, but can be modified to any font that includes bold and italics).
The modification involves replacing these two lines in the current script…
set font of text of front document to “Helvetica”
set size of text of front document to 13
…with the following lines…
tell text of front document
set (font of every attribute run whose (font contains “BoldOblique” or font contains “BoldItalic”)) to “Helvetica-BoldOblique”
set (font of every attribute run whose (font contains “Oblique” or font contains “Italic”)) to “Helvetica-Oblique”
set (font of every attribute run whose font contains “Bold”) to “Helvetica-Bold”
set (font of every attribute run whose (font starts with “HelveticaNeue” or font does not start with “Helvetica”)) to “Helvetica”
set size to 13
As always, there may be a much better way to do this than the one I’ve posted here.