The upper bellows in a condensor enlarger is used to focus the light source into the enlarging lens pupil. In a condensor enlarger, the light source is formed into an image by the condensors and this image should be located at the diaphragm in the enlarging lens. The upper bellows adjustment is used to move the light source image to the proper position in the enlarging lens, hence the markings that show various film (lens) sizes for its position. The light source isn't a true point source, so its image formed by the condensors isn't precise. The light cone produced by the condensors must be matched with the light cone of the enlarging lens to achieve uniform illumination.
If you replace the condensor with a diffusion box--one option for the 23C then the light source is not imaged and there is no longer any need to adujst the upper bellows. In this case you simply elevate the light source to fill the diffuser.
Condensor enlargers produce the maximum detail owing to their "high contrast" illumination. Diffusion enlargers are softer and more forgiving of film plane defects--scratches, dust, grain, etc. Many people don't like the rather extreme sharpness of the condensor illumination, while it is preferred by others, especially for black and white work.
You cannot modify a 23C for 4x5 because the condensors or the diffusion box diameter is too small to cover the film size. Also, the longer focal length of the 4x5 lenses (135 mm is typical) will not focus because the enlarging lens bellows has insufficient length. Note that when changing lenses, you normally replace the lensboard and lens as a unit. It's not necessary to remove the lens from the lensboard. Lenses either thread into the lensboard or are retained on the board by a "jam" nut.
I recently modified my 23C II for 3 1/4 x 4 1/4 as follows:
1. Buy a spare film holder from e-Bay. Drill out the rivets that hold the circular alignment ring and then mill out the film opening for 3 x 4" allowing for 1/8" all around to hold the film. When used in diffusion mode, the illumination circle will cover all but the corners of the 3x4 opening.
2. Obtain a 4x5 enlarging lens and a couple of 4" square lensboards (or make the boards from 1/8" plate). Obtain ~3-4" of 3" PVC pipe to make a lens mount extension.
3. In the lathe, square up the PVC pipe ends and cut it off to 2 1/2" length.
4. Configure one of the lensboards to accept the 135 mm enlarging lens either with a jam nut or by mounting an appropriate adapter to it.
5a. Mount one of the lensboards in the lathe and bore the center to accept the PVC o.d. (about 3.54") with a press-fit. Epoxy the PVC pipe into the bore opening.
5b. Mount the other lensboard in the lathe and turn off the square to the diameter of the PVC pipe i.d. (~3.040") for a press-fit into the PVC pipe.
6. Blacken the entire assembly with flat-black paint and/or black flocking on the inside of the PVC pipe extension. Install the enlarging lens into the lensboard.
7. The entire lens-extension assembly mounts as does any other lens into the square spring-loaded lens frame. The extra 2 1/2" length allows the 135 mm lens to focus and it covers all but
the corners of the 3x4" film.
That's about all you can do for a 23C enlarger. You cannot get any more coverage than the diameter of the illumination circle. If you want 4x5 then you have to move up to a much larger unit and that means a lot bigger space.