You can't "remove" case hardening. What you can do is anneal the frame to soften it back to its original basic alloy. After it was annealed and then engraved (which looks good IMO) it should've been re-case hardened to apply that hard skin which was there originally for a reason- it provides hardness to minimize wear from the moving parts and provides a modicum of strength to the steel. The fact that it wasn't re-case hardened after engraving shows a bit of irresponsibility on the part of whoever engineered that project, IMO. Case hardening was/is a common way to utilize cheap grades of low carbon steel that didn't have enough carbon in it (and/or other alloying elements) to allow the whole thing to respond to through-hardening via heat treating- a way to keep costs down. The colors we like on case hardened steel are merely a side effect of the process. In fact most industrial carburizing (another word for case hardening) doesn't employ the archaic methods anymore that produces those colors. Then again, a lot of gunmakers purposely use cheap steel and CCH it because a lot of guys demand it on their guns.
That said, I would be interested to know how Turnbull will remove the current "plum" bluing, and then polish the surfaces to prep it for re-case hardening with out softening/blurring the engraving. Pray they don't have to re-cut the engraving after that prep work, which is the traditional protocol in these situations. Perhaps they have perfected an alternative process, I don't know.
All that said, I would be tempted to leave that revolver frame the way it is, unless Dad hated the look of it too. A new nicely fitted pair of grips would go a long way toward spiffing up that gun just in themselves.
