The conversion of a nitrogen atom to a carbon atom does decrease the total number of nitrogen atoms, but it makes about as much difference as removing a teaspoon of water from the Pacific Ocean.
The fact that it has seven protons is what makes it nitrogen.
Since protons and neutrons weigh about the same, the atomic mass of ordinary carbon is 6 6 = 12.
It is called "Carbon-12," which is abbreviated "C." The fact that the atom has six protons is what makes it carbon.
Notice that the farther away the Mass Number gets from 12-13, the faster they break down (The blue numbers indicate half-lives, the time it takes for one half of the atoms in a sample to break down.).
So the farther the carbon is from the norm, the more unstable it is. They have too many neutrons so they breakdown, releasing a beta particle which effectively converts a neutron into a proton.