It's a total YMMV property by property, day by day thing.

I'm a LTT and have been refused an UG many times when they clearly show they have suites available to sell per the App. Often when I show them that on the App the tune changes, sometimes they come up with another excuse. I'd say showing them the Marriott T's&C's will not yield any better results.

I usually start the conversation with 'so what kind of room do you have for me today', and when they say 'oh Mr. Dokes we have a lovely standard King room for you', I say 'are you not able to UG me to a suite today as a LTT?'...sometimes results in 'oh no, we're sold out of suites today'...at which time I pull out my phone and show them the App (which I have already checked) and say 'is that so? it shows here you still have abc, xyz, etc. available'

Bottom line, while it's unfortunate, you often have to fight, for your right, to UPGRADE! Be informed and know what they have available to sell and you're a step ahead.