I'm just looking for anime/manga where a character of an affluent background (upper class, even nobility or royalty) have to adjust to a middle-class or even lower-class lifestyle.

It sounds like a common enough trope, but the most recent example I can think of is Sansha Sanyou (picrel, from 2016).
I suppose there's also TWGOK, as one of the girls in Keima's harem is an ex-rich girl who became very poor.