Back in the early 80s I did something like this.
Call of Cthuhlu had just come out and I wanted to run it (I was running RuneQuest at the time, so the system-switch would be minimal), but I was having a hard time trying to convince my players.
So I tried bait-n-switch...
I told them we were going to play a G-Men versus The Mob game, but that I hadn't decided which group I wanted them to play. They chose to be the G-Men. We used the BRP rules, modified as in early edition of CoC, and off we went.
Session 1 -- standard G-Men action
Session 2 -- Found a mob hangout, but there was something odd about the place
Session 3 -- Found a strange mobster, shot him with a Tommy, but he kept coming until the second G-Man openned up. Investigated -- the guy was just ... wrong. San checks.
Session 4 -- Heard weird chanting at a mob spot, saw several low-lifes, and a strange Egyptian fellow was leaving, obviously having met with the Pseudo-Capone of my game. Battle ensues, but the G-Men find strange books and powders on the site ...
... "Umm, Wombat, this isn't what we signed up for. Y'gotta stop this -- you're creeping us out!"
Thus ended my first attempt at CoC and my only attempt at a bait-n-switch.
I'm sure such a game could be handled well, but few players will stand for it unless you are really, really careful.