Ed Wells got demoted
In season one, Wells has stripes on his uniform that indicate he is a rank below the rank Pete held in later seasons-two steps below sergeant. We see him being disciplined in one episode for his refusal to keep to protocol and taking unnecessary risks. When we see him again a bit later, his stripes are gone. Could be a wardrobe oversight, but it's also a good bet he might've remained a training officer (as he is confirmed as one later) but was stripped of his previous rank. Sometime after the series ended, Officer Malloy gave his life in the line of duty, in a Dying Moment of Awesome.
I just can't see him going out any other way. The incident at the Temple of Peace and Love in the seventh episode of Season 1 heralded the start of a religious schism.
They were quarreling over meditation styles in More Loving Than Thou fashion.