That's more of a fluff thing, really, but I'm glad you asked, Debby. In my mind, once an Unsent creature has become a fiend, that's essentially the end of their existence as the individual entity that they once were. It's spiritual & metaphysical dead-end; they've devolved into madness & chaos, & the only end to their existence as fiends is to be destroyed. Once "killed", a fiend breaks up into (literally harmless) pyreflies. There's no advancement from that state, from what I see. Here's a quote from my essay on the Spiral of Death in Spira (bolded for relevance):
Quote Originally Posted by Zeta Kai View Post
Once per year, an Unsent must make a successful Will save or become Chaotic Evil. The Difficulty Class for this check equals the number of years spent as an Unsent divided by their Charisma bonus (minimum +1). Those that become Chaotic Evil immediately transform into fiends. A fiended undead is a creature with an equal Challenge Rating to the originals Unsent, but can be of nearly any type/subtype (no Construct, Fey, Giant, or Humanoid fiends have ever been reported, however). The specific creature is determined by the environment of where it was turned into a fiend; only fiends native to that environment will be created there (Cold subtype fiends created only in arctic areas, for instance).
So you see, if the original Unsent were more powerful (IE if it had more HD), then a more powerful fiend would be the result. Therefore, there is no real need for an advancement entry.

I'm sure some will disagree with my thoughts on the subject, either on dogmatic or mechanical grounds. I just wanted to explain my reasoning, & prove once & for all that I put far too much thought into this whole project.