I would ask for the upgrade before you even arrive, my booking process normally goes like..
1. E-mail to GM - Ask about what rooms upgrade to as an RA
2. E-mail from GM - Book X get Y etc..
3. Book room
4. E-mail to GM: Many thanks for your help I have booked X room with reservation #Y
5. E-mail from GM: - Thank you Rupert_S I have upgraded your room to a suite (or whatever)