What’s the difference between 吃上饭 and 吃下饭? 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.
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) --&gt; [吃(得)上饭], [吃(不)上饭]; [吃(得)下饭], [吃(不)下饭]