If I say "I'm going to smoke" I would say "我去抽烟", right? Or would I say "我去抽烟了"?
In this case, 了 represents a change, so I suppose 我去抽烟了 makes sense. Any difference in nuance between the two?
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我去抽烟 is the standard way to express "I am going to have a cigarette". You don't need a "了" at all. Adding a "了" emphasizes the fact that you are about to do it RIGHT NOW. Sometimes, it implies that you are asking for others' opinions. For example, “如果没什么事，我去抽烟了.” （If you don't have anything else for me, I am going to have a cigarette now(outside).)
If you are trying to say "I'm going to smoke."
我去抽烟 I'm going to smoke 我去抽根烟 I'm going to have a cig 我出去抽根烟 I'm going to have a cig outside.
我去抽烟了 could be said when someone ask you
"Where were you?" or "What did you do?" or "Where did you go?"
And it means
I was having a cig/ I was smoking.
"了" could imply a finished action when you put it at the end of a sentence.
了 is signifies the "perfective aspect". Note that there's no tense in Chinese language, but aspect. Perfective aspect in this context means a finished action. (Alternatively, experiential aspect also expresses similar meaning, but they're slightly different)
So 我去抽烟了 emphasised that you've smoked. While 我去抽烟 means you're going to smoke with no aspect. (The four aspects of Modern Mandarin Chinese are perfective, imperfective (durative stative and durative progressive), and experiential.)
If you'd use these sentences to tell people what you are going to do right now, I do not think there is any noteworthy difference between them.
But if you'd use them to respond to someone who asks you what you're going to do, you should use the first one (without 了).
Also, as other answers say, you can use the second sentence to describe what you were doing just now.
Adding unnecessary model, such as “了” does make it more casual. for example， 了 means everything will be end soon。 both in tense and modal so if it function as modal word ，it means you have solved almost every problem asked by others and you can do something to relax yourself like to have a cig。。。了 or to play game 了