Civ 7 how does change capital work?

Does anyone know the actual conditions behind what cites can become your new capital between eras? I thought it was just population based, but I just had a city passed over in favour of two with less population, it was a conquered capital but normally those get offered even when I don’t want them to. Does happiness play into it? It might’ve been unhappy during the transition but I don’t know if that’s a factor. And in the past I’ve not had the option at all inexplicably. It’d be nice if someone already knew because trying to intuit how it works has been a tad irritating and I couldn’t find any answers on a quick lookup, though I might just have not looked hard enough.