Moderation is key and also the changes you're making need to fit with YOUR life. It is better to allow yourself to have a diet coke and actually enjoy it than going cold turkey and perhaps falling off the bandwagon as a result of feeling deprived.
I have the occasional diet coke/pepsi but also drink seltzer when I want something fizzy. Do what works for you. It may not be the best beverage ever but if it is replacing regular pop or a calorie laden beer then why not?