When ever I want to upgrade the nibs I have always used Jowo nibs. I don't remember if the Cambridge section is threaded for a Jowo housing. If you purchase a nib with the housing just remove the nib from your new housing and place it in the housing that came with the Cambridge. I have done this with the Cambridge, Majestic and several Barea pens. I get my nibs from Pen Realm. He does a nice job of tuning them before shipping them out. Note his stock is usually limited. I have also purchased nibs from Goulet, both his branded and Edison branded nibs. I don't remember off the top of my head if the Cambridge is a 5 or a 6.
