There's nothing to suggest, at present, that they aren't grandfathering in currently owned properties.
Further, while many are delighted that the *shakefist* accursed house hoarders *shakefist* might lose a house or two, their fixation on that blinds them to one simple truth: Houses will be harder than ever to acquire.
Why? The timer imposed after a plot is relinquished (and presumably auto-demolished). Going forward, you will no longer be able to buy a house from a flipper. You will no longer be able to give (or receive) a house to (from) a friend when you (they) quit. You will not be able to turn your personal house into an FC house (or vice versa). When you spot a newly-demolished house, you're going to have to compete for it.
Why? Because during that imposed timer, anyone looking at the Ward (and anyone they inform) will see that the plot is available to purchase.
Also lol at the bit about them letting you share your house with alts.