I converted my Kona Rove ST to flatbar because I was using it mostly for family rides around the neighborhood (and because I can't help but incessantly tinker with my bikes). Most of the relevant points have been raised already...I switched to a 120mm stem (increase from 90mm stock) with 750mm bars to maintain reach as much as possible. Kept the Rival drivetrain and used a cheap Apex flat bar shifter. I had a set of take-off MTB hydraulic brakes which I swapped in place of the road BB7s that were on there.
My swap wasn't particularly cheap- I bought a Thomson stem to match the seatpost, a new Salsa handlebar, had my LBS shorten brake lines & bleed brakes. The brakes were SRAM Guide RS, which would be expensive if purchased new, too. You could do it cheaper with second hand parts, cheaper new parts, using Shimano R550 flat bar levers to keep road brakes, etc.
And for what it's worth, the bike was fine for laid back neighborhood rides, but would not have been my first choice for dirt, especially anything remotely technical. I now have a Specialized Chisel with a rigid carbon fork and like it a lot more than the Rove.
