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.