I'm not a heating pro but I would wager that they would tell you that you CAN'T mess with these ducts without messing up the balance of your system. If all you need is 2" there are a couple things that you can do from a carpentry standpoint to "workaround" this problem. The following cheats or shortcuts are not orthodox. However, it is common to cheat and fudge in cramped basements and we are talking about interior partitions with no loads on them. 1. You can cut 2" off the bottom of the door and the door frame, thus lowering it. (or) 2. You can just leave off the framing header (2x4 at the top of the rough door frame) and put your finish door frame right up against the duct or the finish ceiling material that you put right up against the duct. this can give you at least 1.5". Good luck.