i do this practically every time i play the game. besides a few dialogs that are tied to their classes, i see the NPC's as just personalities and their class or even race is just a matter of need. it lets me see how a kit plays rather than having to make my pc one. it also lets me have whatever kind of party i want without resorting to the no-dialog/no-romance option of multiplayer.
my current party:
me, elf F/Th
keldorn, dwarf F/C
jaheira, 1/2 elf inquisitor
heirdalis, tiefling bounty-hunter
imoen, elf M/Th
next go around ill make edwin a githyanki wild mage and aerie a vampire monk
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