What’s the difference between 吃上饭 and 吃下饭?
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]