The series you are reffering to is Neomorphs by Capnnerifer. You can find them on FanFiction.Net, and probably the Animorphs Fan Forum.
To everyone: A good post-54 fic to read is Animorphs 55: The Last by Gryphon 124. Honestly, I swear that's the best fanfic I've ever read. You can find it under my favorites on FFNet. If you try searching it, you won't get far.
I have never read the Neomorphs series, but I've heard it's very good.