Here is my modus operandi regarding upgrades:
I usally write something about an upgrade visavi my HHonors status in the comment box when booking. Then the day before I arrive I send the hotel an email stating that I made a serch on the Hilton website and found availability for exec level rooms and then request an upgrade visavi my HHonors status and rules in the hiltons frequent stay program. This has worked good so far... got an upgrade to exec floor as a gold in Shanghai, Beijing and Bangkok and other places... I guess it will not work always but it works sometimes...

/Anders