The future progressive is a different, more vague pattern. I don't enjoy trying to succinctly communicate it.
It's not exactly wrong if you switch the balloons there, but... it would be wrong to say they have the same usage, and when teaching ESL speakers, they need to have some guidelines. If I were to go into class and say they're different but it doesn't really matter, the student wouldn't be very illuminated. Usually I tell them this is the best usage, but there can be overlap, and nobody is likely to misunderstand what you mean if you mix them up. Good teaching practice is to differentiate between mistakes that are a grammatical faux pas and mistakes that obscure meaning. (A student today submitted this plan: He's going to go to the Arc de Triumphe, and then he will probably go to Paris. I let him know he had to check the meaning of will probably again.)
I do think it would sound pretty weird to hold up your lunch and say "I'll have pasta," in 99/100 cases though; the case where you say "I'm going to have..." at a restaurant implies you've made up your mind before telling people, which people do actually do with menus. But for the sake of a classroom setting, this communicates the concept quite clearly, I think. If the student can get that "be going to" generally refers to a plan/situation before the conversation takes place, and will refers to something decided during the conversation, it did the job.