Not sure why you need GREP to do this but if you want to, I think you need two expression (Obi-wan might be able to do it in one)
Problem trying to do it in one expression is that you have two different substitution you want to do.
The look of a quotation-mark depends on the assigned language and therefore on the preference in Dictionary > Language > Double Quotes. The correct way would be: Change the look of the quotations here and then run a find: Shift+2, change: Shift+2 (either text or grep panel) and make sure before, that Preferences > Type > Use Typographers Quotes is checked.
Why ist this maybe a better way than a simple find/change? Because if you type now quotion marks, you will get also the right ones.