It was in 2005. So those still serving in 2005 were given a choice: stay under the old scheme or switch to the new one. One of the significant changes in the new scheme was enhanced dependants' benefits—a higher death-in-service lump sum, higher-percentage pensions, and retention of benefits on remarriage. Those who had already left got the benefits they had.
On March 26th, 2009. See this statement in context.