Rotate Window Shapes but not Windows

I have a layout like this:

   ┌──────────┬─────────┐    
   │    A     │    B    │    
   │          │         │    
   ┼──────────┴─────────┤    
   │         C          │    
   │                    │    
   └────────────────────┘

And I want to rotate the window shapes so that I have:

  ┌──────────┬─────────┐  
  │    A     │    B    │  
  │          │         │  
  ┼──────────┤         │  
  │    C     │         │  
  │          │         │  
  └──────────┴─────────┘

So I want the shapes to change, but the general window positions to stay the same.

For whatever reason, Tangris seems to reset things from the second picture to the first pretty regularly, and I'd love a way to fix it easily. Right now, I drab every window from C into a stack in A, adn then swplit that stack, but it's a pain an I feel that three really should be a better way.

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