I think the idea so far is to add a segwit account to segwit enabled wallets. The segwit account then has segwit addresses (p2sh at first). The user is free to choose between his segwit account and non-segwit account and whether he wants to transfer his funds to the segwit account. When he uses the segwit account, he uses segwit change addresses. So every user can decide for himself if/when he wants to update to segwit (of course, only after it gets activated).
That sounds like foolish wallet construction, IMO. Don't do that. A wallet should use segwit (in which case all newly generated addresses should be segwit) or it shouldn't (in which case none of it is).
As far as bare P2WPKH outputs, indeed-- those could be used for change, but they're more identifiable which is pretty ugly.