if I remember correctly, only sugary soft drinks are being taxed, and in two bands of tax depending on the sugar content, would Coke-a-cola reduce the sugar content of it's 'full-fat' version just to come in on the lower tax bracket? I doubt it, not when they have plenty of other sugar free variants already out there. I can see the accountants crunching some big numbers as to whether to add the tax on to the 'full-fat' and then either reduce can size or incorporate some across the board price increase that's balances out their profit when sales of the 'full-fat' version nose dives.
Lucazade did drop it's sugar content some time ago and has caught out a few insulin dependant diabetics trying to treat hypos.