Don't just sit there. Do it!
Now thumb through it, and look at the NPC capsule descriptions. Notice anything funny? PC race beings have a variety of statistics, mainly level. Fiends, on the other hand are described pretty much by the name of their subrace: "babau," "pit fiend," or whatever.
I, however, think that fiends deserve a degree of distincitveness beyond their subrace--especially where powerful and enigmatic figures are concerned. As far as statistics goes, there's little difference between one of the Dark Eight and any other pit fiend. All role-playing aside, when time comes to come to blows with a fiend, I think the general of Gehanna should be more powerful than a new Ultroloth.
Other than giving legendary fiends the power they deserve, there's two other motivations for this. First is the PC power factor. High level PCs, even if a conservative campaign, can overpower true tanar'ri and greater yugoloths and baatezu. I'm personally fond of the idea of balors and pit fiends that make even a powerful party quake in its boots.
The second is surprise factor. The PCs may think they'll make easy work of a lesser Tanar'ri or Yugoloth. But if the DM slips in an exceptional specimen, the players may be in for more of a fight than they bargained for.
Accordingly, I frequently use the DMs Option: High Level Campaigns rules for "legendary monsters" to adjust fiends (or celestials) of notable status. I do this partially because it makes sense that notable specimens would be - well, notable. I do it partly because for a creature that's supposed to be so damned powerful, a Balor's 13HD doesn't quite cut the mustard agaisnt today's cutting edge PC party.
One might argue that a balor shouldn't go toe-to-toe with a high level party without other fiends around. But it's so much more epic when the party can say "remember that balor we fought! Boy I thought we were all toast! Wow he was powerful!" instead of "What a wuss that balor was - he had to summon a horde of Tanar'ri to fight us off!". Denizens of the netherworld should be threatening in the extreme, IMHO, even to a very high level party.
Now I don't usually apply this haphazardly. "least" specimens usually have no legendary specimens at all (except maybe as comic relief: "I am king of the ddrreeetchesssss!"). Lesser fiends might have lesser scions and scions (though succubi, alu-fiends, cambions and erineyes might occasionally have elders or greater). Greaters fiends might have elders, True Tanar'ri (or the greatest of the Greaters for 'loths and the Baatezu) might have great elders. Paragon fiends tend to be planar lords or gods in thier own rights, though that doesn't mean it can't happen (a villain I made before the advent of High Level Campaigns was a demigod--the cambion offspring of Grazzt called "Paralite"; he was essentially a cambion paragon.)
I use the Hellbound: The Blood War
boxed set's spellcasting fiends rules in addition - but if a fiend has
spellcasting per the Dark of the War book, I count it as one of
thier powers, as spellcasting fiends are somewhat rare. However,
such fiends are truly frightening, because while the Dark of the War
limits fiends spellcasting levels to thier HD or innate spellcasting ability
(whatever is better), the High Level Campaigns legendary creatures
rules give them extra HD - and I
allow that to reflect in thier max level. I figure the general
of Gehanna is a great elder ultroloth with 33 HD, and spellcasting as a
33rd level mage (only MORTAL wizards are limited to 30th level...)