Final Fantasy I - Redux

Sorry for the necro, but have you considered using espers and relics to achieve different class stat gains, sprites, starting stats, and available equipment?  It should be possible to change the equipped espers and relics via event scripts (not a scripter so don't quote me on it) and each of those effects already exist for espers, relics, and other equipment.

Moogle Suit - changes sprite IIRC
Espers - change stat gains
Relics - change stats by both straight value and % value, change equipment options

With these three effects all redirected to Espers and Relics, and a buttload of Espers and Relics currently available, you should be able to do this with a lot of ASM hacking. I don't know if there are enough full commented disassemblies, but with just 4 characters you could remove a lot of stuff and save a lot of room for other things.