Top is the old behavior, bottom is the new one:
(Document is sRGB with linear gamma)
It can change the resulting look quite noticeably. The new behavior with the colors mixing proportionally in the semi-transparent areas is more like I’d expect - even if subjectively a bit more boring looking 
.
I personally don’t have any documents where this change matters, but I can see how it could be good to have some compatibility mode for it.
As for the build-up mode and smudge brushes - I haven’t noticed any difference so far.
Edit: Okay, I can spot the difference painting over low opacity areas on the alpha locked layer with a build-up mode brush.
Here again, the new implementation seems cleaner.