Having recently upgraded a Sora/Tiagra setup to all Tiagra, yes, you'll need to change a lot more than just the brifters; it's a whole 8 to 9 speed upgrade which will require a new cassette, and maybe a new chain, derailleurs, and even chainrings or cranks depending on what you currently have.
You could also check out
this thread which talks about some "generic" Shimano 8-speed brifters that should be a drop-in replacement for your Sora brifters, but has the Tiagra/105/Ultegra-like paddles for shifting. That might be a good -- and cheaper (especially if you are otherwise happy with your 8 speed setup) -- option for you.