# How do protons and neutrons interchange?

I recently read in a book that a nucleon with +1/2 isospin is a proton and if the isospin is changed to -1/2, it is a neutron, so it is the same particle in different states. If a proton has a different quark configuration $uud$ than a neutron $udd$, what happens to the quark composition when the isospin changes from +1/2 to -1/2?

The most important isospin-changing particle in the nuclear force is the pion, a triplet of particles with charge ±1 or 0. The $\pi^±$ are the isospin raising and lowering operators that you're interested in: the reactions $\rm p\leftrightarrow n\pi^+$ or $\rm n\leftrightarrow p\pi^-$ rotate the isospin of the nucleon but preserve quark content.