PS2 F3 ABA to PS3 ?

If you can get the PS3 boards to do so yes.

You can PS-3 upgrade your two powered A units and run as a Lashup//MU.  You could PS-32 upgrade your Lead A and run as is, but not sure the value of that unless to repair a bad board.  G

If you change the original PS2 board to PS3/2, can you still use the original slave board in the trailing A unit, or does the slave board have to be changed to PS3? Or does it depend if you load a PS2 file or upgrade PS3 file set?

Yep, the PS32 board works fine with the PS/2 slave board.  I confess I haven't loaded a PS/3 file into such a configuration, but I'm guessing that wouldn't change the operation.

Could someone explain how the slave unit works? Why did MTH stop producing them in this manner?

PS-2 slave with a PS-32 work fine, in fact the 2 pin jumper on the top is a better motor signal for the PS-2 slave and helps prevent the typical Slave FET failure that can occur. Even with PS-3 sound file.  Just keep PS-32 flash code.

PS-2 slave takes varies signals, but primarily the motor signal and replicates it to drive the motors at the same speed the lead A is driven.

PS-3 got more complicated, in the slave got a data signal to replicate Lead A function.  PS-3 slave have a tach reader on a motor, so you have more precision between units.  But it does require the harness connection between units which is a pain.  Plus a separate line of parts for the slave A.

I guess with a majority running dcs, it was simpler to go to 2 powered A units and run them as a MU.  No harness connection.  G