Could you list what parts you have to change to convert a Model 2 to Model 3 cabinet ?
The biggest difference is that Model 3 requires 3.3V power (similar to newer JVS boards) while Model 2 only uses 5V so the power supply will need to be upgraded to a model that provides a 3.3V power source.
There are really 2 connections that need to be changed when going Model 2->Model 3.
1. The Main Power Connector:
Model 2 has 3 10-pin connectors for 5V power, 5 pins for 5V, 5 pins for ground on each of these (CN1, CN3 and CN12 on M2A and CN9, CN10, and CN11 on M2B/M2C).
Model 3 uses one of these 10-pin connectors for 5V power (CN14) and also has a large 24 pin connector (CN13) that includes 8 3.3V lines, 6 5V lines and the rest of the pins are ground. you could re-pin one of the 5V connectors from model 2 to populate half of this connector and then fill out the rest of it with fresh lines for 3.3V. In my case I'm leaving the two extra 5V connectors from Model 2 disconnected and simply adding a whole new connector for the 3.3V/5V connectors that Model 3 requires so that I can easily swap back to Model 2 if I so choose.
2. The Fan Connector:
Model 2 has an externally mounted 12V case fan that uses a 2-pin Molex connector going directly to the fan (not in the diagram), Model 3 has 2 internally mounted 12V case fans that receive their power through a 3-pin JST NH connector on the filter board (CN16). For this I made a simple adapter cable with a 3-pin NH connector on one end and a Molex connector on the other end. This makes it plug and play, and keeps me from having to run new 12V wires while reducing the number of unused connectors.
*A Note about the 5V output connectors
Model 2B/2C and Model 3 all use the same 11-pin connector for 5V output (the additional pin makes it easy to tell apart from the 10-pin input connector). Model 2A has a 11-pin 5V output connector too, however Model 2A also has an additional 9-pin 5V output connector. that the later boards are lacking. This seems to be the biggest compatibility difference between Model 2A and 2B/2C.
All other connectors should be pin-compatible.
I will say that the audio connectors were listed has being pinned differently in nearly ever manual I cam across, even the audio connector on one of my Model 2 boards (just one) was plugged in backwards, which I confirmed by metering out the ground pins. so I'm least confident in that connector (though worst case is they L and R are swapped or it's out of phase).
Most pinouts for the 25-pin digital control connector that I found online were wrong when compared against the service manuals and against my own machine wiring. so it would be nice to get confirmation on that connector from another working setup.