I have one of these CD-i controllers:
I wired Up and the C button to a switch so I can use the C button to jump like on Mega Drive games. I also swapped the A & B buttons as (I think) A is normally the main action button.
I numbered some of the pins in red. I lifted pin 18 - This is up, so the black wire is soldered to this pin and goes to the middle of the switch. Blue wire = up on the d-pad - this is soldered to the point on the board beneath pin 18 - wire goes to one end of the switch. Yellow wire = C button - this goes to the other end of the switch.
Pins 1 & 24 are A & B. So to swap them I lifted both pins and wired them to the points on the board.
Where the red line is, there was a piece of metal connecting the two points. This is where the C button normally goes, so I removed it.