They would definitely be the first to go.
I and probably many others especially chose to become a member and build in the member world to have a somewhat more "permanent" place to build. (And I'm still building there every day.)
Never was it said any world would be permanent. You should pick your choices more carefully perhaps.
I didn't say
permanent, I said
more "permanent", as in definitely longer lasting than guest worlds.
When I joined there were only guest worlds and one member world. Of course I chose the member world to have a bit more "permanent" place. If I would be joining now I'd choose the new member world, an option I didn't have back then.
Oh well, at least we'll have a world download in the crazy case member worlds are removed.