How to replace font FAMILY (to keep typefaces intact)

Hi. I want to replace the font FAMILY, not the font itself. Doing the latter would kill all the typeface information. If a paragraph has a mix of typefaces – such a regular, bold, and italics – replacing with a particular font (say,`Optima-Regular’) kills all the bold and italics. So doing the following fails:

tell application id “com.devon-technologies.thinkpro2”
tell selected text of think window 1
set properties to {font:“Optima-Regular”, size:12}
end tell
end try
end tell

Using Optima' instead also fails; AppleScript treats it as a synonym forOptima-Regular’.

So how does one replace the font FAMILY, assuming that this is the method to use?

It’s only possible to replace the font, not the family.