I have an Omega C700 with 50mm enlarging lens that I am attempting to switch to a 90mm lens for a project I'm working on. The only problem is, I can't get this 50mm to move. My understanding is that I should be able to just screw it off. Am I missing anything in this assumption? I did buy this enlarger second hand, so I don't know if there is something different about it than there should be.