To back the repositioning, which will introduce the line 'For those who love life', Coca-Cola is boosting promotional spend by 50% to £10m and adding the words 'Sugar free' to Diet Coke cans.
The brand, which last year exceeded sales of original Coke for the first time, is critical to the soft-drinks giant, as it enables the company to show it offers healthy products.
Julia Goldin, marketing director for Coca-Cola GB, said a change of approach was needed to turn Diet Coke into a lifestyle brand. 'Our ability to say this is sugar-free, has a great taste and is part of an active lifestyle is important,' she added.
The ads, created by Vallance Carruthers Coleman Priest break at the end of the month, featuring an animatronic turtle.
The campaign includes images of people exercising, with one spot using the voiceover 'Live fast, love life and feel good in your shell'.
The revamp will be backed by Coca-Cola's biggest sampling drive to date, with more than 4m cans distributed.