I haven't heard any reports of anyone being able to trade up (either Snack to CS or CS to TS). That's not saying it's impossible, just in the past 3 years that I've been around these parts I have not heard of a single person successfully doing it, and have heard several reports of people being denied.
Trading down does happen from time to time (and quite often during the "free dining" time period), but trading up is a rarity if ever.