you can take a hammer and lightly hit the corner bead and the mud will pop right off of it. then you can see if the bead is just clinched on there or if it was put on with screws. then it should be easy to stick a flat bar behind the drywall that is casing out the window and pop it off and remove any fastners that was holding it. when putting in your wood put in the casing that goes into the window first and make it match up with the drywall that is sticking out the furthest, then put on your piece on the outside.