Buy some coke in resealable bottles and some club soda. First start by having a smaller amount - say 8 oz instead of 12. Then have 6 oz of coke mixed with 2 oz of club soda. This will slowly wean you off both the caffeine and help your taste buds like less sugar. After a bit go to 4 and 4, etc. You'd be amazed at how revolting full sugar sodas will taste if you train yourself to not use as much sugar.

The other option is to try switching to black or green iced tea. It would be doing the sugar cold turkey (although you could always add 1 tsp or so - still WAY less than the amount of sugar in a coke).

You're addicted to caffine. Wean yourself off slowly. After 2 weeks, the headaches will go away.