Are Claris(ST-2400) shifters drop-in replacements for St-2300s? I've got 2300s on my bike. I don't much like the little thumb levers. I'd like to upgrade to Claris shifters, but I need to minimise the costs.

Can I just change the shifters? They're both 8 speed (and double, in my case), but are the cables compatible as well?
Yes I test rode a Sora a few years ago with those thumb shifters - truly awful. 

My gut feel (I know Shimano well, but don't have direct experience of this groupset) is that one is a direct replacement for the other, but the only evidence I can find to back this is a review on Amazon. 

For sure there'll be no problems shifting, because you're staying at 8-speed. I was just a bit hesitant to answer in case something to do with the braking might have changed. But that guy's review suggests you'll be ok - sounds like he did exactly what you're proposing.