Your signature says you aren't going to be at the Fort until Christmas time. We will know long before then if anything significant is going on behind the 300 loop. So, far, it has just been the installation of electric conduit, which has been reported as being nearly complete as of Labor Day.
So, no need to worry about changing your reservation yet.
If there is a major construction project going on, you can be assured that it will be reported here on the Disboards.
There was major work done on the comfort stations, pool and the loop renovations in the past couple of years. From my observation, Disney was very respectful of insulating guests from any noise and inconvenience from this construction, and I would expect the same if anything significant is going on behind the 300 loop.
Also if it is anything really major, I wouldn't expect them to begin work on it until things slow down at the Fort, like mid-January. They aren't going to want to create a ruckus and disturb guests during the busy Christmas time.
TCD