He wouldn't have to change the stem, because it would be the same diameter. (if the bars as a whole need to be lifted, he'd want to change to a stem with more rise, and maybe more reach to keep the bars from crowding the cockpit.)

My moustache bars are on my commuter. I like drops fine (one of my other bikes has drops) but wanted a bit more leverage and height and control (you have a fistful of bar when you hoick on the brakes, and you don't open your hand to shift placement on the bar) for riding in traffic.

The moustache has a bit less variety for hand placement, and it's a bit harder to stretch your back out, but I'm very happy with them. (I even managed to find narrow-ish 50 cm STEEL ones, and the road buzz is just GONE from that bike)

Channlluv- moustache bars look like drop bars that have been run over by a steam roller. The drops are slightly down and very out to the sides, rather than very down and slightly out to the sides. But I agree, it sounds like you are locking your elbows, and it's a good idea to check your fit and your posture before you change bits of the bike.