I'd like to hear from people who've done this modification, what are the pitfalls?
Mostly I want to know about incompatibilities between Road and Mountain/Hybrid brakes & shifters.

The reason I want to do this is I am old and find a more upright position more comfortable.

The bike I have in mind is this 2006 Giant OCR 3 with 3x8 Shimano Sora brifters & Tektro dual pivot brakes. I'd rather not go down the upgrading rabbit hole as I am a recreational rider on undemanding terrain. I like this bike's weight and size, it fits me well, but I prefer the flat bar and upright position.