The condenser units I have are basically spun aluminum cylinders that have two lenses inside, arranged convex-to-convex with a spacer between them. They are intechangeable between the two enlargers.

They are just held on by three keyed lugs. Loosen the thumbscrews, rotate the cylinder about 1/8 of a turn then pull the lift handle to raise the lamphouse. Slide the unit out, put the new one in its place and line it up. Lower the lamp, rotate it back to lock it in and tighten the screws.

I've got two different ones. A 6" dia. and a 4-1/2" dia. which I presume are for 4"x5" and for 6cm. square, respectively.