I've once overheard a US-based property frontdesk manager said to her staff that "we don't upgrade Platinums to suites, ever." Not sure if she meant they only upgrade Ambassador or Titaniums, or it's Platinum or above. I'm Titanium and I didn't get an upgrade during that stay, app shows availability for junior suites.

But to be fair, I guess I'll mention, I was impressed with Boston Sheraton. I got an "double-upgrade" when I book two rooms (one for my colleague). Both of us got a big suite. Kudos to Boston Sheraton!