Expanding closet opening to its fullest
How easy this project is, will depend on the underlying framing.  There's two ways these corners could have been framed.
Side walls to the end
This will make what you want to do fairly easy.

Top down view of the framing
End walls to the side
This is the more likely of the two. Which means you'll either have to build out the corner, or lose 3 1/2&quot; (+drywall thickness) in closet depth. Though I guess you're not really losing anything, from the interior dimensions of the original closet.

Top down view of the framing
Aside from that. As long as the closet is not bearing, it shouldn't be too much work.