Enlarging a fireplace for a new insert

Question:  Enlarging brick fireplace origionally wood converted to gas with-out insert. I am looking at fireplace inserts and most of them require more height for the opening. My current fireplace opening is 43\"W X 25\"H. The insert I am looking at is 42\"W X 30\"H. Can I remove the steel header and a few bricks from the top of fireplace opening with-out having it all come crashing down? And mortor back in the steel header? Or do I look for a different insert?

Jake's answer:  We have installed larger inserts in existing fireplaces.  We used an experienced mason to make the modifications.  Not only will you have to raise the steel lintel but you may have to modify the firebox height as well.  If you are not knowledgeable with fireplaces, I suggest you hire a mason or find a shorter insert.