They didn't change anything about their steel. It's the same stuff they've been using for a long time. They branded it 'Case Tru-Sharp' (sometimes called 'Tru-Sharp Surgical Stainless'), but it's been the same 420HC for at least a couple decades, if not longer. Case has verified this, when asked. There've been a few posts in the Traditional forum about this. From a usability standpoint, the biggest difference I see between Case's version, and Buck's, is that Case's tends to hang on to wire edges more tenaciously, so it takes a little longer to clean those up. And that can easily be attributed to the difference in the heat treat. Otherwise, one gets just as sharp as the other.
The generic 'surgical stainless' moniker is as non-specific as it could possibly be, and has often been applied to literally any stainless steel with relatively high chromium content (beyond the minimum ~12% or so, to be called 'stainless'). It's even been used to describe 300-series steels, often used for plumbing, nuts/bolts/screws, tableware (knives/forks/spoons) and liners/bolsters on many knives. So, if a knife is labelled as just 'surgical stainless', it can imply a huge range of either good or not-so-good quality. I remember a lot of Parker-branded, Japanese-produced knives back in the '90s or earlier that were labelled that way. They were wicked slicers, and I'd bet the steel was very similar to 420HC. Edges on those knives looked & cut very similarly to the edges seen on current Bucks, and sharpen up as easily.