#4 - March 1, 2011, 11:17 p.m.
If you have the achievement you'll keep the title. The Exalted worked differently because we wanted to move the title to a different achievement altogether (50 exalted).
In this case, if you have the achievement you'll keep the title. If you're close to getting the achievement, you might want to hurry and get it before 4.1 because after 4.1 you'd need to also gain the additional rep to meet the requirements, earn the achievement, and get the title.
Same as it was for Loremaster or Explorer, if you had the achievement you still have the achievement, even though the requirements changed.