I once read in an article supporting MWI that when a particle changes state, it changes into all possible states, that each new state of that particle would be within a reality separate from the realities experienced by the particle's other states.

If this were true, then with each change of state of every particle in existence, there not only would there be new realities equal in number to all the possible states of all the particles in the universe, but an exponentially greater number of realities equal to the number of all possible *combinations* of new particle states...and this would occur not only with every change of state of every particle, but would occur in every new reality that had come into being with every particle state-change (and combination thereof) since the Big Bang.

Compared to such a concept, the Big Bang would be small beer on less than an atomic scale. It's enough to make a physicist complain that his brain hertz...frequently (even if he's not a frequentist).