I think one means it hasn’t happened yet and the other means it’s already happened, but I don’t really understand what that means when trying to translate into English or when trying to think of it in some kind of context.
3 Answers
Without context, you can only break down the phrase into word types and follow the grammar rule to interpret it literally
吃(verb: eat) + 上(verb particle: successfully/ able to)+ 饭(noun: meals)
吃上饭 = successfully/ able to eat meal = able to acquire and eat food
To check this structure is universal or not, replace the noun and see if it is still grammatical
吃上龙虾 = successfully/ able to eat lobster = able to acquireand and eat lobster
~
吃(verb: eat) + 下(verb particle: down) + 饭(noun: meal/ food)
吃下饭 = eat down a meal = ingest food
To check this structure is universal or not, replace the noun and see if it is still grammatical
吃下毒药 = eat down poison = ingest poison
Longer sentences can illustrate the functions of the phrases better
1a. 他窮得快吃不上饭了 = He is so poor that he'll soon unable to acquire and eat food (= can't afford food)
1b. 有工作才吃得上饭 = You can acquire and eat food only when you have a job
2a. 他虛弱得吃不下饭了 = He is so weak that he can't eat down food (= can't ingest food)
2b. 他的病好了許多，已經吃得下饭了 = His body has recovered a lot, already able to ingest food
Studying the example, 吃上饭, and 吃下饭 mostly work with potential particles 得 or 不 (able/ unable) --> [吃(得)上饭], [吃(不)上饭]; [吃(得)下饭], [吃(不)下饭]
忙了一天我终于吃上饭了 -> i finally managed to eat after a busy day
this emphasises being able to.
我心情不好，吃不下饭 -> i'm not in the mood to eat
下 emphasises "down"
the minimum requirement of eating is to swallow down the food, but i am not even able to do that because of bad mood.
我去学校的食堂吃下饭 or 我去学校的食堂吃一下饭 -> i'm gonna grab some food from the uni's cafe
in this example, 下 does not really mean "down" it's more of saying this is not going to take long.
take a computer analogy, think of this like a piece of code only costs very few cpu cycles
think of 一下 like a piece of code only costs explicitly 1 cpu cycle
吃下饭：eat
我们还可以在电影开始前去外面吃下饭。
We can have dinner before the film starts.
吃上饭： have something to eat, however meagre, as opposed to nothing at all
他们已经四五天没吃上一顿像样的饭了。 They haven't had a decent meal in four or five days.
