Well, I guess I answered my own question, though I'm still confused as to why it looks the way it does.

If I connect SSMS to (local), it appears to be connecting to the newest instance. The new instance so far has no databases attached, and also has a ReportServer object in object explorer, so I assume that this is the new Enterprise Edition instance.

So to connect to the new instance i choose (local), and to connect to the old instance I choose localhost\sqlexpress

That is the opposite of what I would have thought, given that various things I'd read said that my new instance would be "named" and the old instance would be the general one for the server.

Anyway, I'm up and running, though a little confused as to why it works the way it does. So if anybody has any explanatory insights to offer, that'd be great. Otherwise, I guess I'm good, thanks...