With this mod you can adjust pasture fences on already built barns as often as you want.

In build mode, a new "Customize Fence" button appears for all buildings with a pasture fence.
This allows you to recreate the fence completely, just like when placing a new barn, to create space for more animals or free up building ground.
Afterward, you can optionally plant a new meadow on the new area.

Notes:
- Since you can also shrink the pasture, overcapacity can occur with a completely full barn. This normally has no effect in the game, except that (like with a full barn) no offspring is generated.
- The new pasture fence costs a bit of money per meter, like any fence in the game. If you don't want that, I can recommend the mod "Free Gates And Fences" by thatSepticGuy.