This is a great question! From my understanding, Quantum Field Theory posits that what we think of as particles are actually excitations of the underlying field

This is a great question! From my understanding, Quantum Field Theory posits that what we think of as particles are actually excitations of the underlying fields. So when a neutron decays into a proton and an electron, it doesn't mean that the neutron consists of a proton and an electron; merely that the "neutron field" has a coupling with a "proton field" and an "electron field". So particles can indeed turn into other particles, as long as certain conservation laws are obeyed. [Edit: typo]