The normal time frame for a fleet is about 30 years. Every 15 years or so mechanical components, and over time beyond that 15-year mark structural components, need to be replaced. This is the third refurbishment of that fleet. That's why it will be the last refurbishment. A refurbishment was undertaken in the late 1990s and early 2000s, again in the 2007 time frame, and now this refurbishment.
On February 24th, 2016. See this statement in context.