Yes, you are basically forced to change your shifters and crankset. If you need that triple, then if you change the crankset to a modern compact double, you may not have enough range in the rear.
I think that when power meter manufacturers fit a PM to a new crank, they need to validate the PM's readings for that type of crank in house, plus do any other work that's specific to that crank. The 6703 is a bit of a different shape than the more modern cranks. It's technically possible that you could convince 4iiii (the only remaining crank based power manufacturer that retrofits your existing crank) to make a meter for the 6703. You will be the only such customer, so they aren't going to do it.
Pedal-based power meters are also an option, and they're much more portable between bikes than crank-based ones. For now, the best option seems to be the Favero Assioma or their new SPD meter, the former of which is typically discounted around Thanksgiving. It also uses Look Keo-style cleats (XPedo cleats, slightly different than the original Keo). The Garmin Rally is similar performance but quite a bit more expensive. I think all the other existing pedal-based PMs right now are similar performance and more expensive.
I've generally found that perceived exertion is sufficient to pace my efforts on the road, although if you are trying to hit a very specific range of power, it may take practice. You can also use heart rate. You might consider getting a smart trainer if you were planning on doing structured intervals - that smart trainer will also take care of your winter riding.
