I did exactly that to an old bicycle which is now long retired. The only step we added was at one point we tried to heat up the original aluminium handle bar to see if we could change the angles
enough to be better ergonomically - not good enough to be worth the effort!
Eventually I replaced that bike with not only the upright posture, but with a lower cross beam so that it was safer if I needed to jump off in a hurry.
For me it was neck pain, rather than low back, but pain wasn't worth a little less wind resistance.