I think you can only convert a neighbourhood into a subhood if it itself has no subhoods. Unless I misunderstand what you're asking.

Does a custom hood, with 5 custom subhoods, Bluewater Village, a custom Uni, the three vacation hoods, all the bin Sims and townies, as well as my own CAS sims qualify as a megahood?

I'd call it one.

Technically, Oakbrook is an uberhood (as it contains all the Maxis main hoods and then some), but I find megahood a more pleasing word and call it that instead. I'm a bit strange though.

If I recall, you have more than 100 households in all those custom hoods, subhoods.
I don't think that's a megahood, maybe more along the lines of an uber-megahood? Or maybe a mega-uberhood?

@Ihate, yes, that is exactly the kind of thing I was wondering. I makes sense that a hood with subhoods cannot become a subhood, but I probably would not have thought about it until it was too late.

I'm nowhere near ready to covert my neighborhood. When I do, I'll likely make it a shopping district since my Sims won't be playable if I convert it to a Vacation destination.

Oh..does that mean my main Hood Aridia could not become my other main Hood's shopping district? Since it also has uni and such...
That's way too complicated for the trouble, you know..:/

Yeah, I got one mega and one uber. My only Uber is gender-swapped though .

I love them. Especially coming up with new Maxis Sim pairings.