I'm not ratting out anybody, but someone I know, does this a lot. He happens to be my father-in-law. He usually gets away with it and sometimes the FAs just let him remain in the undeserved business class seat. He always pulls the old and confused look when in fact he's as sharp as a casino boss.

Here's a few tips from him...

(1) dress casual, but nice, like you're going to a cruise
(2) answer all questions slowly after some thinking
(3) pretend you don't know the difference between C and Y
(4) beg... very few FAs have the stomach to turn down a
requst like that from a little old man...

His all time best record was sitting in C for the entire flight from NY to London last year.