I don't have a studio, just basing this on what I've read here.
Doesn't the studio actually have TWO "slots" for "drives" inside?
(not really "drives" in the traditional sense of the term, but high-speed flash-mem cards)
If so (or even if there's only one), the drives could be "removable" or "replaceable", but I'm going to guess that if a drive IS removed or replaced, it will also require some special software/firmware application (by Apple) to integrate it with the "unified CPU" design of the m-series CPU.
I doubt this software/firmware will be made available by Apple.
So... any future "upgrade" will need to be done BY Apple.
And they probably won't come cheap.
I wouldn't be surprised to see a similar or same design used in an m-series Mac Pro.