You can and I have many times replaced only the windows int the bow unit and leave the bow itself. This would most likely save some serious $ but the bow structure must be in good shape with little or no rot. To make the exterior look good and maintenance free your installer must be excellent with a brake. Id also ad that the top of the bow that has water damage is most likely not due to a leak but the seat and header on those are thin laminated wood and the damage has been from sweating/condensation over the years.
Q