The Valhallan conundrum

I'm having trouble finding a compelling hook for the back story of my Valhallans.

There was once a time that I had the Valhallan 42nd Mountaineers, a regiment mostly of foot troops, led by the zealous Colonel Zedowsky. Over time, my regiment changed in its nature. Elements of support came from the 135th Krieg and some Irregulars that the regiment adopted from their time on Utica IV, a desert planet that had some of the most vicious mountain warfare.

Now, it is lead by a different model, one Captain Sorin (as Colonel Z had some wargear that was costly for a model unlikely to survive actual combat...). Captain Sorin doesn't believe in front line fighting, and is much more likely to be poring over a data-slate and directing his men from his Chimera Katya than being on the front line.

The nature of the regiment changed even more. The 135th didn't make as many appearances. The Irregular's veteran duties fell back on the 42nd, who enjoyed their new Valkyrie. The Mordant 57th Penal Legion was attached to them for mine clearing duty, and the received the support of an Astropath and of a Squad of Psykers. Medusas and Leman Russ Demolishers started making more appearances. The question I have for myself is if they are still the Valhallan 42nd any more? Do I see it as the same regiment with changes, or as an entirely new body?

Maybe that's why I'm having a hard time writing for them, unlike my other armies. Of course, I haven't written anything for my Flesh Tearers. Stelek (of YTTH infamy) first suggested my army list to me, and I feel like they're kind of soulless. I haven't played them as much for that reason.