I like it. The idea of a lost planet returning to the fold and the Investigation teams discovering abhuman, yet loyal, inhabitants. It's a really cool way to use beastmen models, I'd love to see the conversions you get out of these. I can't wait for you to really flesh out a fantastic idea like this. Just thought I'd say:
1. Your numbers are a little iffy. 10,000 years seems a bit too long, as the population would have only been recently discovered. Not long enough for them to become that dedicated. Planets are lost through warp storms all the time, so you really can make up any stretch of years you want. As for the makeup of the fighting forces you don't really need to say. A suitably vauge answer like "hundreds of regiments, with many ogryn and ratling auxiliary units, have been raised from Khai-zhan" is perfect.
2. If there was no Chaos taint discovered, how is the change explained? (and is it family safe?

)
3. A single planet, no matter how unique, is unlikely to warrant the direct attention of the High Lords of Terra.
4. The Imperium is notoriously distrustful of abhumans. The ruling class is likely to be pure human, and the regiment might field a lot more commissars (also human) than usual. This is a perfect opportunity to let the grimdark unfairness of the Imperium play out here.
Fantastic stuff, champ.