I looked at all the kits on PSI's page, didn't find a premium classic. found them at woodturninz I think. The section looks to be a standard 5mm, but I could not tell from the pic if it is the 3 piece all plastic version consisting of the nib, feed, and a single cast plastic body with the gold ring on the end, or if it was the better 4 piece style with an enameled brass housing, a plastic section that threads into that holding the nib and feed. In any case, you should be able to remove the nib and feed together by holding them as close to the gold ring as you can and pulling with a little twisting back and forth action to break them loose. If that don't work, you might try soaking it in some warm (not hot) water for a while to loosen it up. Most of the time there is a flat spot keying off the bottom of the feed, so the nib and feed will not spin inside the housing, but a little rocking pressure while pulling on the nib and feed together will usually work them out.
To reassemble with new nib, turn the nib upside down, position the feed on it, hold together, locate the flat spot inside the section (if there), and insert into section as a unit. Usually they will fit in tightly enough that the nib will not move around on the feed without quite a bit of pressure. If it don't, slide the nib a little farther out on the feed, insert till the feed bottoms out, then push the nib back until it is in the correct position and hope it's tight enough to stay there. If not, you may have to play musical components till you get a combination that fits right. Extreme cases may benefit from sacrificing a hamster to the Great God Doolang, patron Deity of aging blues musicians and fiddlers with obsolete writing equipment. YMMV
