Warbird questions

A customer of ours has a P-51D that we are painting. Right now it's painted as "Ain't Misbehavin" but we are redoing it more like the original. As far as we know this is not the original plane so I was trying to use the serial number to find out the real history of this particular aircraft. This leads to my questions. The AAF serial number of the plane is 44-74009. The first two numbers are the year the plane was built, correct? And once a plane has that serial number, can it be changed or does it stay with that airframe forever like a VIN on a car? The earliest info I can find on this plane is a build number and delivery date in January of 1951. The history goes all the way up to one of the four present day owners. So was this plane built in 1944 or in early 1951? If it was built in 1944, what was it doing for the seven years before it was delivered to RCAF?