(sorry for the not really academic English, this is not my native language)
 
I dissassembled my newly bought Switch 10 to check if the HDD is replaceable. Good news, it is.
 
First, you have to unscrew all cross-head screws on the back of the keyboard, do not forget to remove the rubber piece to unhide 2 screws.
So, you'll be able to dissassemble the two main part of the keyboard.
 

 

 

 
 
Victory ! We can see the HDD !
 

 
4 screws attached te HDD to the chassis...
 
Unscrew these one
 

 
And these last two one
 

 
Now, the HDD is nearly released, there is only one proprietary cable to remove :
 

 
Gently emove the black tape
 

 
You can see that the HDD is standard :
 

 
Now you have disconnected the HDD from the cable, you can install your new HDD/SSD
 

 
My Samsung 850EVO installed
 

 
You will notice that thet keyboard is in fact a USB hub, let see how the HDD is recognized :
 

 
Hope it will help :-)