[villagetinker]

It sounds like you need to widen the window opening, my understanding is that you will need a new lintel beam, as well as cutting the new opening and the window. A neighbor had something similar done (window converted to doorway) as part of a larger project, I believe this change was several thousand dollars several years ago. Original neighbor has passed away, so I cannot verify my memory.
You should be able to contact some of the local construction firm to get a rough idea of the cost. I would also ask the local buidling department if an alteration of the new birdcage (additional door?) would work.