It just depends. I did not have to change my stem, or rather I did but only because the clamp diameter of the trekking bars is 25.4 and the clamp diameter of my flat bar is 31.8. Shims would have worked but the local shops stock stems for sale and claim not to have any shims. If it is not windy I prefer an upright riding position. The shorter reach of the rear part of the trekking bars and the fact that I angled them up slightly gives me the upright position I was hoping for as well as the wind cheating position on the font of the bars. Trekking bars have as many hand positions available as drop bars, you are never very far from the brake levers, and they can be adjusted through a wide variety of settings so they are likely to be adaptable to your needs. I don't suppose they are for everyone but the odds are pretty good that you will be happy with them.

Ken