Disassembling gas fireplace vent

Please help! We are reinstalling our existing gas fireplace and need to change the vent on the back of the fireplace from an elbow and 7" straight, to only the 7" straight to direct vent out of the house. We have tried everything we can think of to remove ( by using bruit force and twisting) the existing vent. It does not seem to want to budge at all. Does anybody who knows about gas fireplace installation know what the trick is to removing this vent??? Please )))