Well that's wonky... really there's some nice precident for fallen celestials retaining some or all of their prior form rather than turning into fiends. Some have become fiends yes, but that seems to be an intentional transformation on their part, like in Baalzebuls case, not something incidental.
There's that fallen Trumpet Archon who rules a city in one of the lower planes who retains his former shape, there's Zalatian XXIII who fell in 2e and who showed up fallen in 3e (I'll have to see what they listed him as there), there's a fallen solar that features in 'Squaring the Circle', etc. None of them transformed into fiends, and for some of the fiend types it's not possible to transform into them unless you start out as one of them (I don't see fallen Guardinals becoming Yugoloths or Demodands/Gehreleths).