Just got a upgrade to a presidential suite at a full service Marriott property tonight in the US! I always check to see what they have available right before I check in. I usually say " I just checked online and saw that the junior suite's are still available can I get upgraded to that room?" I will say if available I get upgraded 90% of the time. Now I do arrive around 8pm normally so I don't know if that makes a difference and they are always 1 night stays.