I just purchased a new modle that "breaths" through the back and is made for "through the wall" but I currently have a unit that looks like it was put in when this place was built in the late 60's. I have measured and the new one will fit but the one that currently sits in the hole will not budge. I do not have a clue about these things and figured it should be easy enough to slide one out and the new one in. Any suggestions would be great!!!