No. GREP styles do not change text, they apply a (character) style to text.
You can’t do it with a GREP style, but you can use GREP when you find & replace and replace/apply/remove a character style with the very same step and save it as a find & replace routine.
Hm, wouldn't that be possible using a special font created by IndyFont?
Of course that would be an "optical" illusion. One single character depicting two single quote shapes made by IndyFont. Could be a problem, if someone else likes to copy/paste the text down the workflow road.
So GREP find/replace is the way to go…
In fact, I think that create a special font with these two glyphes is finally a clever idea. If I needed to do it, I would do it using IndyFont, a char style and a grep style!
@Obi-wan – it might be too clever…
Depending what distribution chanels you are using after layout, it could backfire to use a special font.
Print would be ok. DPS app building as well.
When we come to HTML, ePub or PDF for downloading and reusing the trouble could begin.
In eBook reader software the one character with two single shapes would be possibly substituted with a single double quotation mark. Depends on the reader and the font the user is chosing.
Well, PDFs uploaded to a website, where the user expects to copy/paste text from the PDF, there could be some surprises. Or workflows that work together with a database on XML basis where text is pulled out and fed to from InDesign this would not work as well, if other tools are linked to the database like websites that pull text from the database. Or HTML export from InDesign through in5 software, just to mention this. Hard to foresee, if the fonts can be used on the web.