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).

It would be possible to use a mixed account with both segwit and non-segwit addresses and maybe even native p2wpkh addresses for change.  But this may be confusing to users, cause inconsistent balances between different wallets (e.g. when sharing xpubs), and make it easier to spot the change output.