The "connecting" piece, i.e. in addition to permissions, can only be achieved thus far by some form of linky type navigation integration. Intertwining the menus on one site and the other, giving the appearance of merging the two, sort of thing.
The one suggested above was to use Teams. A Team has its intrinsic new OG site. And this is the site that the link on the Team tab called "Files" goes to. But you can add another tab to the Team called Old Files and link that to the old classic SP site.
Then on the left nav / quick launch of both old and new sites put all the same menu items / links on both old and new sites. You could indent the old and new libraries on the menu one above the other. So the Contoso scenario might look like this: -
V Documents
Old Documents
V Finance Docs Library
Old Finance Docs Library
If the user clicks on "Old Documents" they're taken to the "Old Documents" library on the old site. But because the left nav / quick launch menu looks identical on both old and new sites, they don't perceive the click through as a site-to-site navigation. Rather the old library looks like a sub-folder of the new library, and the click through appears as merely navigating to the sub-folder in the same site. (That visual trick / technique is quite useful for when you hit the 5000 limit on libraries too, splitting out the biggest root level folders into their own libraries and indenting them below the original library on the left nav / quick launch from whence they came).
And if the permissions setup is done as above and doesn't hinder them, they might, as one user said to me, wonder if the migration has already been done: "Why did you bring the old folders over into a sub-folder of the new library? Couldn't you have copied them to the root?"
No, of course, the users, or me, or someone has still got to lift those folders across. This is not a "magic merge" or groupification / teamification upgrade of an old "classic" SP site. That's what we're waiting for from Redmond. But at least Teams, the menu interleaving, and permissions tricks gives the perception of progress, integration, and old-new hybrid-ity quite early on in the project.
On the subject of the so-and-so's getting the actual job done, looks like they've missed the "this year" deadline. I suspect it might be quite a bit trickier than they thought. The new OG site and old "classic" site are two totally different SP site templates. The project to deliver the new OG sites was probably all done in an Agile/ Scrum-ish sort of way, with no-one lifting their head above the tactical parapet of the new OG world user story backlog to ascertain what strategic chaos it might cause for migrating / upgrading from the old world.
Go on Redmond, prove me wrong. Tell me you can "magic merge" or groupify / teamify / upgrade an old "classic" SP site on patch Tuesday next week! ...
