Paul I and msmith point out the reasons why adding Roth to an IDP formatted VS plan via a short amendment is difficult. While not impossible, I'd say that it would be less expensive to restate the plan than to create an amendment that modifies a large number of sections of the document. As far as the impact on reliance, I wouldn't worry because it would just need to be a good-faith amendment. When the plan is restated for the next cycle you get retroactive protection. But again, much less expense to restate the plan.
As to whether an amendment to add Roth is needed now, that is definitely the safe way to go. In the past, the IRS has provided that even discretionary provisions of a law are integrally related to a change in the qualification requirements. I would expect the IRS to do that here - lots of new discretionary options. No telling if that would be broad enough to cover the addition of the underlying Roth provision. But as Paul pointed out on the sequencing, etc., restating the plan to address those issues now might be worthwhile, even if the IRS does provide for broad retroactive amendment relief.