It's utterly rare, but it happens. Like it or not, fiends can be redeemed and celestials can fall. Baalzebul went LG to LE, Asuras are archons that fall along the Law/Chaos axis, and I can think of several neutral or good fiends.
But the question is: is a risen fiend still a fiend? Yes and no. Sort of. A fallen archon certainly isn't an archon, they're something new entirely, and similarly for a redeemed fiend. For a true outsider, their alignment = their soul = their physical body. If their alignment were to shift, their physical composition and appearance is going to change just as radically as their behavior and psyche will.
3e has not exactly handled this well, or in detail. WotC had a succubus paladin, but they kept her [Evil] subtype, which makes little sense. We don't keep the [Good] subtype on fallen archons, be they Zalatian XXIII or Baalzebul/Triel.
And then of course there's the Sanctify the Wicked spell (aka conversion by the sword, aka Exalted mindrape), but let's try to avoid that, because as is, most paladin threads go downhill fast enough as it is.