Seems to me it would be very easy to wall in the rest of that coop/run to accomodate twice as many birds and just add a large wire run for foraging.
this is what's I'd do instead of trying to tear out something. Just wall it up and then make an external run.
the original 'it's good for 10 chickens' seems to be an industry standard for 'bought' coops and plans. They exaggerate the number it will hold to get your money