I'm not usually a fan of recasting, unless it's absolutely necessarily. The radicaly difference of Neral between "Unification" and the aforementioned episode (referred to, from here on, as IAESL), however, is one of the strangest.
Neral in Unification was explicitly described as a relatively young leader. He looks round faces and bright. Just seven years later, however, we get this extremely thin, aged fellow who looks a couple of decades older. Look, I know being a political leader is a stressful job that ages people prematurely, but come on!
Why they didn't go with Norman Large again is total mystery. The only explanation is a cryptic respsonse from Ron Moore saying "We did consider it, but decided to go in another direction
", whatever the hell that means. I doubt they thought he was a bad actor, since they cast him in three other roles after Neral.
If they simply wanted a more seasoned character leading the Empire in the episode, why not simply create a new one?