It's not too hard. In fact, we can swap positions on both our tandems. Our newest tandem can be converted by putting on a longer stem up front, grabbing the long alloy seat post for me, attaching the adjustable stoker stem to it and putting my wife's seat post and saddle on the back. It puts us a bit further apart and my wife would have to shift blind since she wouldn't be able to see around me, but we could ride this way just fine. In the two plus years we've had this bike we have not bothered to do this, although we have all the parts. Even though my torso/arms are quite a bit longer than my wife's, some of that difference is mitigated by the fact that I prefer my handlebars a bit below my saddle and she likes hers a bit above her saddle.

We still have our first tandem from 1988. It's a snap to switch back and forth as we just switch the front stem.(It was upgraded to 1 1/8 threadless headset four years ago, so stem changes are easy.) However, the stoker compartment has the bars a bit close to the saddle for me, which causes me to sit up higher and be less comfortable. On that bike, we can easily change the shifting to down-tube shifters for the captain, but the stoker still handles the ancient Phil Wood disk brake. Although we freely switched places in the first few months of riding that bike, we haven't done so in years. Even when we did, we just left the shifting to the stoker.