Ask them what version they stock prior to ordering? I currently do the opposite when I ask for the sugar free version usually.
I would do usually, but it's been happening at my 'regulars' (including pubs). So I'm used to having full fat. One pub I went to changed my drink twice, after taking it back and explaining it tasted strange, before they admitted to not stocking full fat anymore.
I'm not a fizzy drinker really, only when I have a meal.
Today at Subway, they wouldn't swap the dispensed drink for a bottle, or even give me a water to have. I didn't end up drinking anything, but I'd blinking paid £1.30 for it.
If you ask for a Coke and they give you a Diet Coke then ask for a refund.
Now, if you ask and they say "sorry sir, we only have Diet..." then that's different. I prefer Pepsi Max and rarely have full-fat sodas because I'd rather not 200 calories in a drink
I was at the dispensing machine when I asked if they no longer did full fat, and he said "Oh yeah we don't because they're not healthy". I did ask for a refund but he said he couldn't. I think had it not have been a cold Sunday, I'd have argued with him, but I just wanted to get home.