John,
Welcome to the Thorn/Rohloff/‘straight handlebars’ club.
It will be interesting seeing what stem/handlebars combination you end up with.
Let your hands/wrists/arms/shoulders/back guide you into a comfortable position.
Andre has some interesting views on this.
Julian.
Thing is, for people of a certain age, unless some cutoff event makes it impossible for them to maintain the position on the bike they've been in for a lifetime, they're better off sticking to it because there will be a period of pain involved in changing it.
But if you must, or strongly desire to move away from drops, I don't think flat bars are the bee's knees, not even the Thorn flat bars.
If you need or want to sit up straighter, you need to bring the grips closer to you and a shorter stem will only bring the grips towards you a little distance; flat bars are no help, quite the contrary.
You might also want to move the seat back if you're sitting more upright, another factor that isn't helpful in the reach department.
A good all-round solution is North Road bars, which offer several sets of good ergonomics, like using them upside down as moustache bars or, right way up, to bring the bicycle's chief control closer to your hands in a natural position. Also, they offer a change of grips and of posture on the bike, similar to drops. I like the Uno Kalloy North Road bars, which are at the cheap end of the market. They've served me well for decades on all my bikes. Uno Kalloy also makes a stem that is toollessly adjustable on the road (at a standstill, not on the move!), by just throwing an over-centre lever, rotating the handlebars and locking the lever again. That permits the same handlebars to offer positions from down and forward for a flat back, to up and forward like wider aero grips.
With a Rohloff, any HGB with a rotary control, it is important to get North Road bars with unshortened grips, so that you have a good straight piece of grip for your hand and for mounting the rotary Rohloff gear change and the brake lever, and whatever else you want to mount, like perhaps a rotary concentric bell on the other side, light switches, computer and so on. If you have such a long straight grip, your hand moves just slightly to change gear. You don't want to ride with part of your palm on the rotary control if you can help it.
North Road handlebars should be set up with a downwards slant to the grips, not to a fixed formula but where your wrists are happiest.
One more important point. We speak loosely of sitting "upright", but nobody should sit truly upright because that is a sure recipe for damaging your back. Instead one assumes a slight forward bend at the hips, at least enough to take the coccyx and backbone out of the direct line of the seat slamming upwards in the same line when riding through unexpectedly pothole or whatever. 7-15 degrees might be suitable; more is semi-sporting and from 30 degrees is definitely sporting.
I would say to someone changing riding positions to take his time and give each adjustment enough time to become familiar and, as Julian says, to let his body tell him what to do.
