On the basis of what we've seen so far, I don't think this is right. It's new tenancies, created after 1 March 2026, that will be six-year rolling tenancies with more limited grounds on which the landlord can terminate. I don't read the announcement as saying that existing tenancies will automatically be converted on 1 March. That suggests that it's only when an existing tenancy expires that a six-year tenancy of the new type will be created, if the property is re-let.
But, having said that. all we have at the moment is a media announcement set out in bullet points. JoeRoberts is right to say that we need to see the draft legislation. It's very common that, when new policy is announced, not a lot of thought has been given to the details of how the transition from the old policy to the new will work. (Or, if they have thought about that, they don't bother to explain their thinking in the press release.) So this is something to keep a close eye on.