It may be that your fireplace guy is trying to "protect" you from the details, taking a "leave it to me" attitude. I can tell you with confidence that some re-framing will be needed. Some modifications to your existing chase will be required to accommodate a new high-efficiency fireplace and chimney. The clearance codes are much more strict than for your old "builder box" type of fireplace. It would not be unusual for me to charge an extra day of labor to do the re-frame and get it inspected by the local building inspector. Don't assume it's a quick swap.