How easy it is to switch to diet depends a bit on how much you drink. For a time I was drinking a bunch of coke, so switching to diet was initially very rough, but also went quickly, maybe a couple weeks. Having free access to diet coke at work sped things along.

Now if I drink regular US coke (the HFCS kind) it feels like I'm drinking syrup, and my mouth feels like it's coated in glue.